MB 557: Systemic & Diagnostic Bacteriology-I

Medical Microbiology — M.Sc. Semester II

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Systemic & Diagnostic Bacteriology-I

These notes focus on the laboratory diagnosis of important bacterial infections. The diagnosis is presented from the clinical suspicion and specimen collection stage through microscopy, culture, identification, antimicrobial susceptibility testing and interpretation.

Special attention is given to the correct specimen, timing of collection, patient preparation, collection technique, transport, specimen quality and the difference between normal flora, colonization and true pathogens.

Contents

  1. General Approach to Bacterial Diagnosis
  2. Upper Respiratory Tract Infections
  3. Lower Respiratory Tract Infections
  4. Urinary Tract Infections
  5. Gastrointestinal Bacterial Infections
  6. Cholera
  7. Enteric Fever
  8. Gonorrhoea
  9. Syphilis
  10. Gas Gangrene
  11. Wound and Soft Tissue Infections
  12. Helicobacter pylori Infection
  13. Tuberculosis
  14. Bacterial Meningitis
  15. Bacteremia and Septicemia
  16. Antimicrobial Susceptibility Testing
  17. Interpretation and Reporting
  18. Complete Laboratory Diagnostic Workflow
  19. Long-Answer Examination Format

1. General Approach to Bacterial Diagnosis

Laboratory diagnosis of bacterial infection does not begin with culture. It begins with proper clinical assessment and correct specimen collection. A good laboratory result depends heavily on the quality of the specimen received by the laboratory.

The same organism may be a pathogen in one specimen but a normal colonizer or contaminant in another. Therefore, the specimen source, clinical findings, collection method and laboratory findings must be considered together.

Basic Diagnostic Sequence

1. Clinical suspicion: The clinician identifies the likely infection on the basis of symptoms, signs, history and examination.
2. Selection of specimen: The specimen should come from the actual site of infection whenever possible.
3. Timing of collection: Collection should be performed at the appropriate stage of disease and, whenever clinically feasible, before antimicrobial therapy.
4. Patient preparation: The patient should receive appropriate instructions depending on the specimen.
5. Collection: The specimen is collected using an appropriate aseptic technique and suitable container.
6. Labelling: The specimen container must be correctly labelled and accompanied by relevant clinical information.
7. Transport: The specimen should reach the laboratory promptly under the conditions appropriate for that specimen.
8. Laboratory receipt: The laboratory checks patient identification, specimen type, container, volume, labelling, leakage, timing and other acceptance criteria.
9. Processing: The specimen is processed according to its type and suspected infection.
10. Microscopy: Direct examination may provide rapid information about inflammatory cells, organisms and specimen quality.
11. Culture: Appropriate culture media and atmospheric conditions are selected.
12. Identification: Isolates are identified using colony morphology, microscopy, biochemical, antigenic, automated or molecular methods as appropriate.
13. Antimicrobial susceptibility testing: Clinically significant isolates may be tested for susceptibility to appropriate antimicrobial agents.
14. Interpretation and reporting: Results are interpreted together with specimen quality, clinical information and accepted laboratory criteria.
Important: Treatment of a seriously ill patient should not be delayed solely because specimen collection is pending. Whenever clinically feasible, specimens should be obtained before antimicrobial therapy because prior treatment may reduce culture yield.

2. Upper Respiratory Tract Infections

Introduction

Upper respiratory tract infections involve structures such as the nose, nasopharynx, pharynx, tonsils, sinuses and laryngeal region. Most are viral, but several important bacterial infections require laboratory investigation.

Important Commensals and Normal Flora

The upper respiratory tract normally contains a variety of microorganisms. Their presence in a throat or upper airway specimen does not automatically indicate disease.

Important Bacterial Pathogens

Clinical Manifestations

Specimens

Throat Swab Collection: Complete Procedure

Clinical suspicion: The patient has clinical findings suggesting bacterial pharyngitis, tonsillitis or another upper respiratory bacterial infection.
Timing: Whenever possible, the specimen should be collected before antimicrobial therapy. If antibiotics have already been started, this should be recorded.
Patient preparation: Explain the procedure to the patient. The patient should be positioned so that the throat can be examined safely. Avoid unnecessary contact with the tongue, teeth and cheeks during sampling.
Collection: Using an appropriate sterile swab, gently depress the tongue and swab the tonsillar areas and posterior pharyngeal wall, especially areas showing inflammation or exudate.
Important: Avoid touching the tongue, cheeks, teeth and other heavily colonized surfaces as far as possible because this can affect interpretation.
Container: Place the swab immediately into the appropriate sterile transport system.
Labelling: Record patient identification, specimen type, anatomical site, collection date and time and other required identifiers.
Clinical information: Provide symptoms, suspected diagnosis, antibiotic history and other relevant information.
Transport: Transport promptly according to the laboratory's approved specimen transport procedure.

Laboratory Processing

On receipt, the laboratory checks specimen identification, specimen type, transport system, collection information and specimen condition.

The swab may be inoculated onto appropriate culture media depending on the suspected pathogen and laboratory protocol.

Streptococcus pyogenes

S. pyogenes is an important cause of bacterial pharyngitis.

Microscopy: Gram-positive cocci arranged mainly in chains or pairs.

Culture: It produces small colonies, often with beta-haemolysis on blood agar.

Identification:

Corynebacterium diphtheriae

When diphtheria is suspected, the laboratory should be informed because special culture and toxin-related investigation may be required.

A throat or nasopharyngeal specimen should be collected appropriately and transported according to laboratory protocol.

Exam point: Do not write that every throat isolate is pathogenic. Upper respiratory specimens contain normal flora, so clinical findings and the organism isolated must be interpreted together.

3. Lower Respiratory Tract Infections

Introduction

Lower respiratory tract infections involve the trachea, bronchi, bronchioles and lung tissue. Important bacterial conditions include bronchitis, bronchopneumonia, lobar pneumonia, aspiration pneumonia, hospital-associated pneumonia and tuberculosis.

Laboratory diagnosis of lower respiratory infection depends greatly on obtaining a specimen that represents the lower respiratory tract rather than saliva from the mouth.

Important Bacterial Pathogens

Normal Flora and Commensals

The upper airway and oral cavity contain numerous organisms that can contaminate an expectorated sputum specimen.

Therefore, recovery of an organism from sputum does not automatically prove that the organism is causing pneumonia.

Clinical Manifestations

Specimens for LRTI

Depending on the clinical condition, specimens may include:

For routine community-acquired lower respiratory infection in a patient who can produce sputum, expectorated sputum is commonly examined. More invasive specimens may be required when sputum is unavailable, repeatedly poor quality, or when a specific clinical situation requires a lower respiratory tract specimen.

Complete Sputum Collection and Diagnostic Procedure

1. Clinical Suspicion

The clinician suspects a lower respiratory bacterial infection on the basis of clinical examination, symptoms and, where available, radiological findings. The suspected condition should be written on the laboratory request form.

2. Correct Specimen Selection

The specimen should be true sputum from the lower respiratory tract and not simple saliva.

Sputum is material produced from the lower respiratory tract and brought up by deep coughing. Saliva mainly originates from the mouth and does not adequately represent the infected lower respiratory tract.

3. Timing of Collection

Whenever clinically feasible, collect the specimen before antimicrobial therapy. If the patient is already receiving antibiotics, this information should be provided to the laboratory because it may affect culture recovery.

The exact collection timing should be recorded on the specimen or request form.

4. Patient Identification

Before collection, confirm the patient's identity using the laboratory or hospital's approved identification procedure.

The specimen container should correspond to the correct patient and the request form should contain matching identification information.

5. Patient Preparation

Explain to the patient that a deep respiratory specimen is required and that saliva is not suitable.

The patient should be instructed to rinse the mouth with water if required by the local specimen collection protocol. Avoid mouthwash or other substances that may interfere with specimen collection when the laboratory protocol requires avoidance.

The patient should then be encouraged to produce a deep cough specimen.

6. Sputum Collection Procedure

  1. Ask the patient to take several deep breaths.
  2. Ask the patient to cough deeply from the chest rather than simply clearing the throat.
  3. The patient should expectorate the sputum directly into a sterile, wide-mouth, leak-proof specimen container.
  4. The inside of the container and lid should not be touched.
  5. The container should be closed immediately after collection.
  6. If the patient cannot produce an adequate specimen, an appropriate alternative such as induced sputum or another respiratory specimen may be considered according to clinical and laboratory practice.
Important collection point: A thick respiratory secretion produced after a deep cough is preferred. Watery saliva or a small amount of oral secretion should not be accepted as representative sputum simply because it was expectorated into the container.

7. Container

Use a sterile, clean, wide-mouth, leak-proof specimen container suitable for respiratory specimens.

The container must close securely to prevent leakage and contamination during transport.

8. Labelling

The specimen must be labelled according to laboratory policy. Important information includes:

The information on the container must match the laboratory request form.

9. Clinical Documentation

The request form should provide useful clinical information such as:

10. Transport

The specimen should be transported to the laboratory promptly using the approved procedure for respiratory specimens.

Unnecessary delay may allow changes in the relative numbers of organisms and may affect culture recovery and interpretation.

11. Laboratory Receipt

On arrival in the laboratory, the specimen is checked for:

12. Specimen Rejection or Recollection

A specimen may require rejection or recollection according to laboratory criteria if it is:

If clinically appropriate, a new and better-quality specimen should be requested rather than reporting a misleading culture from a poor specimen.

Macroscopic Examination of Sputum

The specimen may be observed for its general appearance before processing. Features such as colour, consistency and the presence of blood may provide supportive clinical information.

Possible appearances include:

Macroscopic appearance alone is not sufficient to identify the causative organism.

Direct Microscopy of Sputum

A Gram-stained smear can provide rapid information about:

Sputum Quality Assessment

This is one of the most important steps in sputum examination.

A good-quality lower respiratory specimen generally contains numerous polymorphonuclear leukocytes when active bacterial inflammation is present and relatively few squamous epithelial cells.

Numerous squamous epithelial cells suggest contamination with oral secretions.

Finding Interpretation
Numerous polymorphonuclear leukocytes Supports an inflammatory process in the respiratory tract.
Numerous squamous epithelial cells Suggests contamination by saliva/oral flora.
Predominant organism seen with inflammatory cells May provide useful evidence when consistent with clinical findings.
Poor-quality specimen with abundant oral epithelial cells Culture interpretation becomes difficult and recollection may be required.
Very important examination point: In sputum diagnosis, do not simply write "Gram stain is done and culture is performed." Explain that the Gram stain also helps assess specimen quality, especially the relationship between squamous epithelial cells and polymorphonuclear leukocytes.

Culture of Sputum

The specimen is inoculated onto appropriate culture media according to the suspected pathogens and laboratory protocol.

Commonly used media may include:

The culture conditions and incubation requirements depend on the suspected organism and the laboratory's validated procedure.

Important Organisms and Their Identification

1. Streptococcus pneumoniae

Microscopy: Gram-positive, lancet-shaped diplococci.

Culture: Alpha-haemolytic colonies may be observed on blood agar.

Important identification features:

2. Staphylococcus aureus

S. aureus may cause pneumonia, especially in selected patient groups.

Further testing may be required to determine antimicrobial resistance, including methicillin resistance where clinically indicated.

3. Haemophilus influenzae

H. influenzae is a small Gram-negative coccobacillus and requires enriched conditions for growth.

4. Klebsiella pneumoniae

5. Pseudomonas aeruginosa

Interpretation of Sputum Culture

Interpretation of sputum culture requires more than identifying a bacterium. The laboratory must consider the quality of the specimen and whether the organism recovered is likely to represent a true lower respiratory pathogen.

Important factors include:

Do not report every organism as the cause of pneumonia. Respiratory specimens may contain colonizing organisms and oral flora. Interpretation must distinguish probable pathogen from contamination or colonization.

Antimicrobial Susceptibility Testing in LRTI

Clinically significant bacterial isolates may undergo antimicrobial susceptibility testing.

The test method and interpretation should follow the laboratory's adopted standard and current organism-specific criteria.

The report should identify the organism and provide susceptibility results in the appropriate format.

Special Diagnosis of Tuberculosis from Respiratory Specimens

When pulmonary tuberculosis is suspected, routine bacterial sputum culture alone is not sufficient.

Specimen

An appropriate respiratory specimen such as sputum should be collected according to the tuberculosis diagnostic protocol.

Collection

The patient should be instructed carefully to produce a deep respiratory specimen rather than saliva. Appropriate infection-control measures should be followed during collection.

Laboratory Tests

The exact number and timing of specimens should follow the applicable laboratory or national tuberculosis diagnostic protocol.

URTI versus LRTI

Feature URTI LRTI
Main site Nose, pharynx, tonsils, sinuses and upper airway Bronchi, bronchioles and lungs
Common specimen Throat or nasopharyngeal specimen depending on infection Sputum or other lower respiratory specimen
Main collection problem Contamination from oral surfaces Saliva mistaken for true sputum
Important laboratory issue Normal upper airway flora may be present Normal oral flora may contaminate sputum
Important quality assessment Correct anatomical site and proper swabbing Presence of inflammatory cells and assessment of squamous epithelial cells

Common Mistakes in Sputum Collection

Long-answer conclusion: The diagnosis of lower respiratory bacterial infection requires a good-quality respiratory specimen. Sputum should be collected properly after explaining deep-cough technique to the patient, placed in an appropriate sterile container, correctly labelled and transported promptly. Laboratory diagnosis includes macroscopic examination, Gram staining and assessment of specimen quality, culture on appropriate media, identification of significant isolates, antimicrobial susceptibility testing and interpretation in relation to the clinical findings. When tuberculosis is suspected, specific mycobacterial investigations must be performed.

Short Exam Sequence

Clinical suspicion → sputum selection → timing → patient preparation → deep-cough collection → sterile container → labelling → transport → specimen quality assessment → Gram stain → culture → identification → AST → interpretation/reporting.

4. Urinary Tract Infections

Introduction

Urinary tract infection may involve the urethra, bladder, ureters or kidneys. Laboratory diagnosis depends strongly on correct urine collection because contamination from periurethral flora can produce misleading results.

Common Causative Organisms

Normal/Contaminating Flora

The distal urethral and periurethral regions may contain organisms that are not responsible for urinary infection. Poor collection technique may therefore produce mixed or insignificant growth.

Clinical Manifestations

Specimens

Clean-Catch Midstream Urine: Complete Procedure

Clinical suspicion: Symptoms suggest urinary infection.
Timing: Collect an appropriate specimen before antimicrobial treatment whenever clinically feasible.
Patient preparation: Explain the procedure. The patient should clean the external genital area according to the approved collection instructions.
Collection: The initial portion of urine is allowed to pass into the toilet or collection receptacle. Without stopping the stream when possible, the midstream portion is collected into the sterile container.
Important: The container must not touch the skin.
Closure: Close the container immediately after collection.
Labelling: Record patient identification, specimen type, date and exact collection time.
Transport: Send the specimen promptly according to laboratory requirements. If immediate processing is not possible, preservation or refrigeration should follow the validated laboratory procedure.

Laboratory Examination

Microscopy

Microscopy may demonstrate:

Numerous epithelial cells may suggest contamination, although interpretation depends on the specimen and clinical context.

Culture

A calibrated loop or another validated quantitative method may be used for urine culture.

The number of colony-forming units is interpreted together with symptoms, patient characteristics and the organism isolated.

Important Organisms

Escherichia coli

Klebsiella pneumoniae

Proteus species

Staphylococcus saprophyticus

Interpretation

Urine culture should not be interpreted only on the basis of bacterial growth. Clinical symptoms, colony count, specimen type, organism identity and the possibility of contamination must all be considered.

5. Gastrointestinal Bacterial Infections

Important Bacterial Pathogens

Clinical Manifestations

Stool Collection

Clinical suspicion: Determine whether the illness is likely to require bacterial stool investigation.
Timing: Ideally collect an appropriate specimen early in illness and before antimicrobial therapy whenever clinically feasible.
Patient preparation: Explain that the specimen must not be contaminated with urine, water or disinfectants.
Collection: Pass stool into a clean, dry collection surface and transfer an appropriate amount into a sterile, wide-mouth, leak-proof container.
Labelling: Record patient identification, specimen type, date and time of collection.
Transport: Transport promptly. Appropriate transport medium may be required for certain organisms or when immediate processing is not possible.

Laboratory Diagnosis

6. Cholera

Causative Organism

Vibrio cholerae, particularly toxigenic strains belonging to important epidemic serogroups, causes cholera.

Clinical Manifestations

Specimen

Collection and Transport

Timing: Collect as early as possible in the illness and before antimicrobial therapy when clinically feasible.
Collection: Collect fresh stool into an appropriate sterile, leak-proof container. Avoid contamination with urine, water or disinfectants.
Rectal swab: If stool is unavailable, a properly collected rectal swab may be used.
Transport: Transport promptly. When delay is expected, an appropriate transport medium should be used according to laboratory protocol.
Labelling: Record patient identification, specimen type, collection time and relevant clinical/epidemiological information.

Laboratory Diagnosis

Important Identification Features

7. Enteric Fever

Causative Organisms

Clinical Manifestations

Blood Culture: Complete Procedure

Clinical suspicion: Suspect enteric fever from compatible clinical findings and epidemiological history.
Timing: Blood culture is particularly important early in illness and should ideally be collected before antimicrobial therapy when clinically feasible.
Patient identification: Confirm patient identity before venepuncture.
Aseptic preparation: Disinfect the venepuncture site and blood culture bottle septum according to approved blood culture collection procedure.
Collection: Collect the required amount of blood using the approved technique and appropriate blood culture bottle.
Blood-to-broth ratio: An appropriate blood-to-culture-medium ratio is important because excessive blood may affect recovery and small blood volumes may reduce sensitivity.
Labelling: Record patient identification, collection date and exact time, specimen type and relevant clinical information.
Transport: Transport promptly to the laboratory according to the blood culture system's requirements.

Culture

Automated blood culture systems may detect microbial growth and signal the laboratory for further processing.

When growth is detected, the bottle is processed for Gram staining, subculture and identification.

Identification

Antimicrobial Susceptibility Testing

Clinically significant Salmonella isolates should be tested according to the applicable laboratory standard and current resistance concerns.

Other Specimens

Stool culture may be useful in selected stages and epidemiological investigations. Urine or other specimens may have a role in particular clinical circumstances.

8. Gonorrhoea

Causative Organism

Neisseria gonorrhoeae

Clinical Manifestations

Specimens

Collection

Timing: Collect before antimicrobial therapy whenever clinically feasible.
Patient preparation: Explain the procedure and collect from the appropriate anatomical site.
Collection: Use the appropriate sterile swab or validated specimen collection device. Collect adequate material from the infected site.
Labelling: Record patient identification, anatomical site, date and time.
Transport: Transport rapidly because N. gonorrhoeae is relatively fragile and culture requires appropriate conditions.

Laboratory Diagnosis

Microscopy

Intracellular Gram-negative diplococci within polymorphonuclear leukocytes can be highly suggestive in an appropriate symptomatic urethral specimen.

Identification

9. Syphilis

Causative Organism

Treponema pallidum

Clinical Stages

Specimens and Diagnosis

Direct detection methods may be useful in selected lesions, while serological testing is the major diagnostic approach in many clinical settings.

Serological Testing

Tests may be divided broadly into:

The interpretation depends on the clinical stage, previous infection, treatment history and the particular testing algorithm used by the laboratory.

10. Gas Gangrene

Important Organisms

Clinical Manifestations

Specimen Collection

A deep tissue specimen or aspirated material is preferred over a superficial swab because superficial swabs may recover colonizing organisms rather than the organisms responsible for deep infection.

Collection Procedure

Urgency: Suspected gas gangrene is a medical emergency. Laboratory communication should be prompt.
Specimen: Collect deep tissue, aspirate or other appropriate material from the infected area.
Avoid superficial contamination: Superficial wound swabs may not adequately represent deep tissue infection.
Anaerobic handling: Use appropriate anaerobic collection and transport methods.
Labelling: Record patient identification, specimen source, collection date and time.
Transport: Transport rapidly under appropriate anaerobic conditions.

Laboratory Diagnosis

Clostridium perfringens

11. Wound and Soft Tissue Infections

Common Pathogens

Specimen Selection

Aspirate or deep tissue is generally more useful than a superficial swab for deep wound infection.

Collection

Laboratory Diagnosis

12. Helicobacter pylori Infection

Clinical Association

Helicobacter pylori is associated with chronic gastritis, peptic ulcer disease and increased risk of certain gastric malignancies.

Specimen Options

Diagnosis may be performed using invasive or non-invasive methods.

Invasive Methods

Non-invasive Methods

Gastric Biopsy Collection

Clinical indication: Patient is investigated for H. pylori-associated disease.
Collection: During upper gastrointestinal endoscopy, appropriate gastric biopsy specimens are obtained for the intended test.
Labelling: The anatomical site and patient identification must be correctly recorded.
Transport: Transport the biopsy according to the specific test being requested because different investigations require different handling conditions.

Rapid Urease Test

H. pylori produces urease, which hydrolyses urea and changes the local pH. This biochemical property is used in rapid urease testing.

Culture

Culture requires specialized conditions and is particularly useful in selected cases where antimicrobial susceptibility information is required.

13. Tuberculosis

Causative Organism

Mycobacterium tuberculosis complex

Clinical Manifestations

Specimens

Respiratory Specimen Collection

The patient should be instructed to produce a deep respiratory specimen rather than saliva. Appropriate infection-control precautions should be followed.

The specimen should be labelled with patient identification, specimen type, date and time and transported according to the tuberculosis laboratory protocol.

Laboratory Diagnosis

Acid-Fast Staining

Mycobacteria have a lipid-rich cell wall that allows them to retain certain primary stains despite acid-alcohol decolorization.

Molecular Diagnosis

Rapid molecular methods can detect Mycobacterium tuberculosis complex and may also provide information about important drug resistance markers depending on the assay used.

Culture

Mycobacterial culture is more sensitive than direct smear microscopy but requires specialized laboratory facilities and appropriate biosafety procedures.

14. Bacterial Meningitis

Important Pathogens

Specimen

Cerebrospinal fluid is the major specimen for laboratory investigation of bacterial meningitis.

CSF Collection and Handling

Clinical suspicion: Suspect meningitis based on fever, headache, neck stiffness, altered consciousness or other compatible findings.
Collection: CSF is collected by lumbar puncture using an aseptic technique by trained clinical personnel.
Labelling: Record patient identification, specimen type, collection time and relevant clinical information.
Urgent transport: CSF for suspected bacterial meningitis should reach the laboratory urgently.

Laboratory Examination

Important Gram-Stain Patterns

Organism Typical finding
Neisseria meningitidis Gram-negative diplococci
Streptococcus pneumoniae Gram-positive lancet-shaped diplococci
Haemophilus influenzae Small Gram-negative coccobacilli
Listeria monocytogenes Small Gram-positive bacilli/coccobacillary forms

15. Bacteremia and Septicemia

Important Organisms

Blood Culture Collection

Blood culture is highly dependent on correct collection technique. Contamination by skin flora can lead to false-positive results.

Clinical suspicion: Suspect bloodstream infection from fever, chills, hypotension, sepsis or other compatible findings.
Timing: Blood cultures should ideally be collected before antimicrobial therapy when clinically feasible.
Patient preparation: Confirm identity and prepare the venepuncture site using the approved antiseptic procedure.
Collection: Collect the recommended blood volume using aseptic technique.
Multiple sets: Multiple appropriately collected sets may improve recovery and help distinguish true bacteremia from contamination.
Labelling: Record collection time and venepuncture site where required.
Transport: Send promptly to the laboratory.

Laboratory Diagnosis

Contamination

Some organisms, particularly common skin flora, may represent contamination rather than true bloodstream infection. Interpretation requires consideration of the organism, number of positive bottles or sets, time to positivity, clinical findings and presence of intravascular devices or other risk factors.

16. Antimicrobial Susceptibility Testing

Purpose

Antimicrobial susceptibility testing determines the in-vitro response of a bacterial isolate to selected antimicrobial agents.

Common Methods

Disc Diffusion Method

  1. Prepare an appropriate standardized bacterial suspension.
  2. Inoculate the appropriate susceptibility-testing medium.
  3. Apply antimicrobial discs according to the validated procedure.
  4. Incubate under the required conditions.
  5. Measure zones of inhibition.
  6. Interpret the measurements according to the applicable current standard.

MIC

The minimum inhibitory concentration is the lowest concentration of an antimicrobial agent that prevents visible growth under the defined test conditions.

Important Resistance Mechanisms

AST should not be performed or interpreted in isolation. The organism identity, specimen source and clinical significance of the isolate must first be considered.

17. Interpretation and Reporting of Results

A microbiology result should answer a clinical question rather than simply list every microorganism detected.

Factors Affecting Interpretation

Pathogen versus Colonizer versus Contaminant

Term Meaning
Pathogen An organism that is capable of causing the infection and is supported by the clinical and laboratory findings.
Colonizer An organism present at a body site without necessarily causing disease.
Contaminant An organism introduced into the specimen during collection, handling or processing and not responsible for the patient's infection.

Reporting Should Include

18. Complete Laboratory Diagnostic Workflow

For a long-answer question asking for laboratory diagnosis of a bacterial infection, the following sequence provides a logical structure.

1. Clinical suspicion

↓

2. Select the correct specimen

↓

3. Decide appropriate timing of collection

↓

4. Prepare and identify the patient

↓

5. Collect the specimen correctly

↓

6. Use appropriate container

↓

7. Label specimen with required information

↓

8. Provide clinical information

↓

9. Transport promptly under appropriate conditions

↓

10. Laboratory receipt and acceptance/rejection

↓

11. Macroscopic examination where applicable

↓

12. Direct microscopy

↓

13. Culture using appropriate media and conditions

↓

14. Colony morphology

↓

15. Gram stain and organism morphology

↓

16. Biochemical/antigenic/automated/molecular identification

↓

17. Antimicrobial susceptibility testing

↓

18. Interpretation

↓

19. Final reporting

19. Long-Answer Examination Format

For a 10-mark or long-answer question on the laboratory diagnosis of a specific bacterial infection, the answer should not consist only of the name of the culture medium and biochemical tests.

Recommended Structure

  1. Introduction: Briefly define the infection and mention the important causative organisms.
  2. Clinical manifestations: Mention the major signs and symptoms.
  3. Specimens: State the correct specimen and explain why it is appropriate.
  4. Timing: Mention collection before antimicrobial therapy whenever clinically feasible.
  5. Patient preparation: Explain relevant preparation and instructions.
  6. Collection procedure: Describe how the specimen is actually collected.
  7. Container: Mention the appropriate sterile or specialized container.
  8. Labelling: Mention patient identification, specimen type, date and exact collection time.
  9. Transport: Mention prompt transport and any important specimen-specific requirements.
  10. Receipt and rejection: Mention important criteria for accepting or rejecting an unsuitable specimen.
  11. Microscopy: Describe the relevant staining and expected findings.
  12. Culture: Mention appropriate media and relevant culture conditions.
  13. Colony morphology: Describe important colony characteristics.
  14. Identification: Give the major biochemical, antigenic, automated or molecular tests.
  15. AST: Mention susceptibility testing for clinically significant isolates.
  16. Special diagnostic methods: Include molecular, antigen, toxin, serological or other special tests where relevant.
  17. Interpretation: Explain how the laboratory result is distinguished from contamination, colonization or normal flora.
  18. Conclusion: End with a short statement linking specimen quality, laboratory diagnosis and clinical interpretation.

One-Line Memory Sequence

Clinical suspicion → Correct specimen → Timing → Patient preparation → Collection → Container → Labelling → Transport → Receipt/rejection → Microscopy → Culture → Identification → AST → Special tests → Interpretation → Reporting

Final Examination Tip

For specimen-based questions, always ask yourself: "Where is the infection, and does my specimen actually represent that site?"

This is particularly important for sputum, urine, wound swabs and respiratory specimens because contamination and colonization can significantly affect the interpretation of culture results.