Section 2 — Medically Important Bacteria
This section covers the major medically important bacteria included under MB 557. Organisms are grouped according to their major microbiological characteristics and clinical importance.
A. Aerobic / Microaerophilic Gram-negative Bacteria
- Campylobacter
- Helicobacter
- Brucella
- Legionella
- Neisseria
- Moraxella
- Acinetobacter
- Bordetella
- Pseudomonas
B. Facultative Gram-negative Rods
- Salmonella
- Shigella
- Yersinia
- Vibrio
- Aeromonas
- Escherichia
- Citrobacter
- Enterobacter
- Klebsiella
- Proteus
- Haemophilus
C. Gram-positive Cocci
D. Gram-positive Spore-forming Bacteria
Introduction to Medically Important Bacteria
Medically important bacteria are bacterial organisms capable of causing disease in humans or contributing to clinically significant infections. They differ in morphology, cultural characteristics, biochemical properties, virulence factors, pathogenic mechanisms and antimicrobial susceptibility.
Identification of clinically important bacteria requires a combination of direct microscopy, culture, biochemical testing, antigen detection, molecular methods and antimicrobial susceptibility testing.
A. Aerobic / Microaerophilic Gram-negative Bacteria
This group includes medically important Gram-negative bacteria that require aerobic or reduced-oxygen conditions for growth and are associated with respiratory, gastrointestinal, systemic, hospital-acquired and zoonotic infections.
Campylobacter
Important cause of bacterial gastroenteritis, particularly Campylobacter jejuni.
Study topic ↓Helicobacter
Helicobacter pylori is strongly associated with chronic gastritis and peptic ulcer disease.
Study topic ↓Neisseria
Includes important human pathogens such as N. gonorrhoeae and N. meningitidis.
Study topic ↓Moraxella
Associated with respiratory tract infections, especially Moraxella catarrhalis.
Study topic ↓Pseudomonas
Pseudomonas aeruginosa is an important opportunistic pathogen and cause of healthcare-associated infection.
Study topic ↓Campylobacter
Morphology
- Gram-negative curved or spiral-shaped rods
- Motile by polar flagella
- Non-spore-forming
- Often described as having a gull-wing appearance
Cultural Characteristics
- Microaerophilic organism
- Grows best under reduced oxygen conditions
- Campylobacter-selective media may be used
- C. jejuni grows well around 42°C
Clinical Manifestations
- Acute diarrhea
- Abdominal pain
- Fever
- Inflammatory or bloody diarrhea may occur
- Post-infectious complications may occur
Laboratory Diagnosis
- Stool specimen collection
- Direct microscopy where appropriate
- Selective culture under microaerophilic conditions
- Identification by biochemical or molecular methods
Exam Focus
- 5 marks: Laboratory diagnosis of Campylobacter infection.
- 10 marks: Discuss Campylobacter with emphasis on morphology, pathogenesis, clinical features and diagnosis.
Helicobacter
Important Characteristics
- Curved or spiral Gram-negative rod
- Highly motile
- Urease positive
- Microaerophilic
- Colonizes gastric mucosa
Virulence Factors
- Urease
- Flagella
- Adhesins
- Cag-associated factors
- VacA-associated effects
Clinical Importance
- Chronic gastritis
- Peptic ulcer disease
- Increased risk of gastric adenocarcinoma
- Association with gastric MALT lymphoma
Laboratory Diagnosis
| Method | Example |
|---|---|
| Invasive | Biopsy-based tests, rapid urease test, histology and culture |
| Non-invasive | Urea breath test, stool antigen test and selected serological methods |
Exam Focus
- 5 marks: Virulence factors of H. pylori.
- 5 marks: Laboratory diagnosis of H. pylori infection.
- 10 marks: Discuss H. pylori infection with pathogenesis and diagnosis.
Brucella
Important Species
- Brucella melitensis
- Brucella abortus
- Brucella suis
- Brucella canis
Transmission
- Unpasteurized milk and dairy products
- Direct contact with infected animals
- Occupational exposure
- Inhalation of infectious aerosols
Clinical Manifestations
- Undulant fever
- Weakness and fatigue
- Sweating
- Arthralgia
- Hepatosplenomegaly may occur
- Focal complications may develop
Laboratory Diagnosis
- Blood culture
- Serological tests
- Molecular methods in selected settings
Legionella
Important Characteristics
- Gram-negative bacillus
- Requires special culture conditions
- Associated with aquatic environments
- Can survive and multiply within amoebae
Clinical Forms
| Condition | Main Features |
|---|---|
| Legionnaires' disease | Severe atypical pneumonia with systemic symptoms |
| Pontiac fever | Self-limited febrile illness without typical pneumonia |
Laboratory Diagnosis
- Urinary antigen testing
- Culture on appropriate selective medium
- PCR or other molecular methods
Neisseria
Neisseria are Gram-negative diplococci. Two major pathogenic species are N. gonorrhoeae and N. meningitidis.
| Feature | N. gonorrhoeae | N. meningitidis |
|---|---|---|
| Main disease | Gonorrhoea | Meningitis and meningococcemia |
| Transmission | Sexual contact | Respiratory droplets / close contact |
| Capsule | Absent | Present |
| Glucose | Fermented | Fermented |
| Maltose | Not fermented | Fermented |
Important Virulence Factors
- Pili
- Outer membrane proteins
- IgA protease
- Lipooligosaccharide
- Capsule in N. meningitidis
Exam Focus
- 5 marks: Laboratory diagnosis of gonorrhoea.
- 5 marks: Virulence factors of Neisseria.
- 10 marks: Compare N. gonorrhoeae and N. meningitidis.
Moraxella
Moraxella catarrhalis is an important respiratory tract pathogen, particularly in patients with underlying respiratory disease.
Clinical Importance
- Otitis media
- Sinusitis
- Exacerbation of chronic obstructive pulmonary disease
- Lower respiratory tract infections
Important Laboratory Features
- Gram-negative diplococci
- Oxidase positive
- Non-motile
- May produce β-lactamase
Acinetobacter
Important Characteristics
- Gram-negative coccobacillus
- Non-motile
- Oxidase negative
- Aerobic
- Can survive on environmental surfaces
Clinical Infections
- Ventilator-associated pneumonia
- Bloodstream infections
- Wound infections
- Urinary tract infections
- Healthcare-associated infections
Laboratory Diagnosis
- Culture and colony characteristics
- Gram staining
- Biochemical identification
- Automated identification systems
- MALDI-TOF or molecular methods where available
Bordetella
Virulence Factors
- Pertussis toxin
- Adenylate cyclase toxin
- Tracheal cytotoxin
- Filamentous hemagglutinin
- Pertactin
Clinical Features
- Catarrhal stage
- Paroxysmal stage
- Convalescent stage
Laboratory Diagnosis
- Nasopharyngeal specimen
- PCR
- Culture using appropriate specialized media
- Serological testing in selected situations
Pseudomonas
Important Characteristics
- Gram-negative rod
- Aerobic
- Motile
- Oxidase positive
- Non-fermentative
- May produce characteristic pigments
- Can grow in a wide range of environmental conditions
Virulence Factors
- Exotoxin A
- Elastase
- Phospholipase
- Pyocyanin
- Alginate
- Type III secretion system
- Biofilm formation
Clinical Infections
- Hospital-acquired pneumonia
- Burn wound infections
- Urinary tract infections
- Bacteremia
- Otitis externa
- Eye infections
- Infections in immunocompromised patients
Laboratory Diagnosis
- Gram staining
- Culture
- Oxidase testing
- Pigment production
- Biochemical identification
- Automated or molecular identification when required
Exam Focus
- 5 marks: Virulence factors of P. aeruginosa.
- 5 marks: Laboratory diagnosis of Pseudomonas infection.
- 10 marks: Discuss P. aeruginosa under morphology, virulence, pathogenesis, clinical features and diagnosis.
B. Facultative Gram-negative Rods
Facultative Gram-negative rods include several medically important members of the Enterobacterales and related organisms. They are associated with gastrointestinal disease, urinary tract infections, septicemia, wound infections and other clinical conditions.
Salmonella
Morphology and Characteristics
- Gram-negative rods
- Usually motile by peritrichous flagella
- Non-spore-forming
- Facultative anaerobes
- Usually non-lactose fermenting
Clinical Manifestations
- Enteric fever
- Gastroenteritis
- Bacteremia
- Focal infections in susceptible individuals
Laboratory Diagnosis
- Blood culture in enteric fever, especially early disease
- Stool culture
- Biochemical identification
- Serological tests in selected settings
- Antimicrobial susceptibility testing
Exam Focus
- 5 marks: Laboratory diagnosis of enteric fever.
- 10 marks: Discuss Salmonella with emphasis on pathogenesis and laboratory diagnosis.
Shigella
Shigella are non-motile Gram-negative rods that primarily cause intestinal disease, particularly bacillary dysentery.
Important Species
- Shigella dysenteriae
- Shigella flexneri
- Shigella boydii
- Shigella sonnei
Virulence Factors
- Invasion factors
- Type III secretion system
- Shiga toxin in S. dysenteriae type 1
Clinical Features
- Fever
- Abdominal cramps
- Diarrhea
- Blood and mucus in stool
- Tenesmus
Laboratory Diagnosis
- Stool specimen collection
- Culture on selective and differential media
- Biochemical identification
- Serological identification
- Antimicrobial susceptibility testing
Yersinia
Medically important Yersinia species include Y. enterocolitica, Y. pseudotuberculosis and Y. pestis.
| Species | Major Disease |
|---|---|
| Y. enterocolitica | Enterocolitis and pseudoappendicitis |
| Y. pseudotuberculosis | Enteric and systemic disease |
| Y. pestis | Plague |
Laboratory Diagnosis
- Culture of appropriate clinical specimens
- Biochemical identification
- Serological or molecular methods where appropriate
Vibrio
Morphology
- Gram-negative curved rods
- Motile
- Comma-shaped appearance may be seen
- Oxidase positive
Virulence Factors of V. cholerae
- Cholera toxin
- TCP
- Other colonization-associated factors
Clinical Features
- Profuse watery diarrhea
- Vomiting
- Severe dehydration
- Electrolyte imbalance
Laboratory Diagnosis
- Stool or rectal swab specimen
- Enrichment where appropriate
- Selective culture
- Biochemical identification
- Serological confirmation
Exam Focus
- 5 marks: Virulence factors of V. cholerae.
- 5 marks: Laboratory diagnosis of cholera.
- 10 marks: Discuss cholera under morphology, pathogenesis, clinical features and diagnosis.
Aeromonas
Aeromonas species are aquatic Gram-negative rods associated with gastrointestinal and extraintestinal infections.
Clinical Importance
- Acute gastroenteritis
- Wound infections
- Septicemia
- Infections in immunocompromised patients
Laboratory Features
- Gram-negative rods
- Oxidase positive
- Facultatively anaerobic
- Can grow in aquatic environments
Escherichia
Major Diarrheagenic Groups
| Pathotype | Major Disease / Feature |
|---|---|
| ETEC | Watery diarrhea, including travelers' diarrhea |
| EPEC | Diarrhea, particularly in young children |
| EHEC/STEC | Bloody diarrhea and possible HUS |
| EIEC | Invasive dysentery-like illness |
| EAEC | Persistent or watery diarrhea in some settings |
Extraintestinal Infections
- Urinary tract infections
- Septicemia
- Neonatal meningitis
- Wound and other infections
Exam Focus
- 5 marks: Diarrheagenic E. coli.
- 5 marks: Laboratory diagnosis of E. coli infection.
- 10 marks: Discuss pathogenic E. coli and its diarrheagenic pathotypes.
Citrobacter
Citrobacter species are members of Enterobacterales and may cause opportunistic infections, particularly in hospitalized patients.
Clinical Importance
- Urinary tract infections
- Bloodstream infections
- Respiratory infections
- Healthcare-associated infections
Laboratory Identification
- Culture
- Biochemical reactions
- Automated identification systems
- Antimicrobial susceptibility testing
Enterobacter
Enterobacter species are opportunistic Gram-negative rods that are important causes of healthcare-associated infections.
Clinical Infections
- Urinary tract infections
- Pneumonia
- Bloodstream infections
- Wound infections
Important Laboratory Consideration
Antimicrobial susceptibility testing is important because Enterobacterales may possess inducible or acquired resistance mechanisms.
Klebsiella
Important Characteristics
- Gram-negative rod
- Prominent capsule
- Non-motile
- Lactose fermenting
- Facultative anaerobe
Clinical Infections
- Pneumonia
- Urinary tract infection
- Bacteremia
- Liver abscess in selected cases
- Healthcare-associated infections
Virulence Factors
- Capsule
- Adhesins
- Lipopolysaccharide
- Siderophores
Exam Focus
- 5 marks: Virulence factors of Klebsiella.
- 5 marks: Laboratory diagnosis of Klebsiella infection.
- 10 marks: Discuss K. pneumoniae under morphology, virulence, clinical manifestations and diagnosis.
Proteus
Important Characteristics
- Gram-negative rods
- Highly motile
- Swarming growth may occur
- Urease positive
- Non-lactose fermenting
Clinical Importance
- Urinary tract infections
- Catheter-associated infections
- Urinary stone formation
- Wound infections
- Septicemia
Important Virulence Factor
Urease production increases urinary pH and contributes to the formation of certain urinary stones.
Exam Focus
- 5 marks: Swarming phenomenon of Proteus.
- 5 marks: Laboratory diagnosis of Proteus infection.
Haemophilus
Important Characteristics
- Small Gram-negative coccobacilli
- Requires specific growth factors
- Chocolate agar is commonly used
- Some strains are encapsulated
Clinical Infections
- Meningitis
- Epiglottitis
- Pneumonia
- Otitis media
- Sinusitis
Laboratory Diagnosis
- Direct microscopy
- Culture on chocolate agar
- Factor requirement testing
- Antigen detection in selected cases
- PCR and other molecular methods
Exam Focus
- 5 marks: Growth requirements of H. influenzae.
- 5 marks: Laboratory diagnosis of H. influenzae infection.
- 10 marks: Discuss H. influenzae with emphasis on virulence and laboratory diagnosis.
C. Gram-positive Cocci
Gram-positive cocci include several important human pathogens. They are responsible for skin infections, respiratory infections, invasive infections, urinary infections and other clinical conditions.
Peptostreptococcus
Anaerobic Gram-positive cocci involved in polymicrobial infections.
Study topic ↓Micrococcus
Micrococcus species are Gram-positive cocci commonly found as part of the normal skin microbiota. They are generally considered low-virulence organisms but can occasionally cause opportunistic infections.
Important Characteristics
- Gram-positive cocci
- Often arranged in tetrads or clusters
- Catalase positive
- Strictly aerobic
- May produce yellow or other pigmentation
Clinical Importance
Opportunistic infections may occur particularly in individuals with underlying disease or indwelling medical devices.
Staphylococcus
General Characteristics
- Gram-positive cocci
- Arranged in clusters
- Catalase positive
- Facultative anaerobic
- Non-motile
- Non-spore-forming
Important Species
| Species | Major Importance |
|---|---|
| S. aureus | Major pathogenic species |
| S. epidermidis | Device-associated and opportunistic infections |
| S. saprophyticus | Urinary tract infections |
Virulence Factors of S. aureus
- Protein A
- Coagulase
- Capsular components
- Adhesins
- Hemolysins
- Leukocidins
- Enterotoxins
- TSST-1
- Exfoliative toxins
Clinical Manifestations
- Skin and soft tissue infections
- Abscesses
- Pneumonia
- Osteomyelitis
- Endocarditis
- Septicemia
- Food poisoning
- Toxic shock syndrome
- Scalded skin syndrome
Laboratory Diagnosis
- Gram staining
- Culture
- Catalase test
- Coagulase test
- Latex-based identification where appropriate
- Antimicrobial susceptibility testing
Exam Focus
- 5 marks: Virulence factors of S. aureus.
- 5 marks: Laboratory diagnosis of S. aureus.
- 10 marks: Discuss S. aureus under morphology, virulence, pathogenesis, clinical manifestations and laboratory diagnosis.
Streptococcus
Major Groups
- Group A Streptococcus
- Group B Streptococcus
- Viridans streptococci
- Streptococcus pneumoniae
- Enterococcus species are discussed separately in many classifications
Important Diseases
- Pharyngitis
- Skin and soft tissue infections
- Pneumonia
- Meningitis
- Endocarditis
- Neonatal infections
- Post-streptococcal complications
Laboratory Diagnosis
- Gram staining
- Culture
- Hemolysis pattern
- Biochemical tests
- Antigen detection
- Molecular methods where appropriate
Exam Focus
- 5 marks: Classification of streptococci.
- 5 marks: Laboratory diagnosis of streptococcal infection.
- 10 marks: Discuss medically important Streptococcus species.
Peptococcus
Peptococcus refers to anaerobic Gram-positive cocci. These organisms may form part of the normal flora but can contribute to opportunistic and polymicrobial infections.
Clinical Importance
- Deep tissue infections
- Abscesses
- Mixed anaerobic infections
Laboratory Diagnosis
Appropriate anaerobic specimen collection, transport and culture under anaerobic conditions are essential.
Peptostreptococcus
Peptostreptococcus species are anaerobic Gram-positive cocci that may be involved in polymicrobial infections involving mucosal surfaces.
Clinical Importance
- Oral infections
- Respiratory infections
- Pelvic infections
- Intra-abdominal infections
- Deep tissue abscesses
Laboratory Diagnosis
- Proper anaerobic specimen collection
- Anaerobic transport system
- Anaerobic culture
- Microscopic and biochemical identification
D. Gram-positive Spore-forming Bacteria
Medically important Gram-positive spore-forming bacteria mainly include Bacillus and Clostridium. Their spores allow survival under unfavorable environmental conditions.
Clostridium
Anaerobic spore-forming Gram-positive rods associated with toxin-mediated and invasive infections.
Study topic ↓Bacillus
General Characteristics
- Gram-positive rods
- Spore-forming
- Usually aerobic or facultatively anaerobic
- Many species are environmental organisms
Bacillus anthracis
- Causes anthrax
- Large Gram-positive rods
- Spore formation
- Non-motile
- Important capsule-associated virulence
Bacillus cereus
- Food poisoning
- Emetic and diarrheal syndromes
- Can cause opportunistic infections
- Usually motile
Exam Focus
- 5 marks: Virulence factors of B. anthracis.
- 5 marks: Laboratory diagnosis of anthrax.
- 10 marks: Discuss Bacillus anthracis in detail.
- 5 marks: Food poisoning caused by B. cereus.
Clostridium
Important Species
| Species | Major Disease |
|---|---|
| C. tetani | Tetanus |
| C. botulinum | Botulism |
| C. perfringens | Gas gangrene and food poisoning |
| C. difficile | Antibiotic-associated diarrhea and colitis |
General Features
- Gram-positive rods
- Spore-forming
- Generally anaerobic
- Many produce potent exotoxins
Clinical Importance
Clostridial diseases are often associated with toxin-mediated pathology. The clinical manifestations depend on the species, toxin produced and site of infection.
Exam Focus
- 5 marks: Virulence factors of C. tetani.
- 5 marks: Laboratory diagnosis of tetanus.
- 5 marks: Botulism and botulinum toxin.
- 5 marks: Gas gangrene caused by C. perfringens.
- 10 marks: Discuss medically important Clostridium species.
Important Bacterial Groups — Quick Comparison
| Group | Representative Organisms | Major Clinical Importance |
|---|---|---|
| Aerobic / Microaerophilic Gram-negative bacteria | Campylobacter, Helicobacter, Brucella, Legionella, Neisseria, Pseudomonas | Gastrointestinal, respiratory, systemic and opportunistic infections |
| Facultative Gram-negative rods | Salmonella, Shigella, Vibrio, E. coli, Klebsiella, Proteus | Enteric disease, UTI, septicemia and other infections |
| Gram-positive cocci | Staphylococcus, Streptococcus, Micrococcus | Pyogenic, respiratory, skin, invasive and opportunistic infections |
| Gram-positive spore-forming bacteria | Bacillus, Clostridium | Anthrax, food poisoning, tetanus, botulism, gas gangrene and other infections |
Section 2 — Exam-Oriented Summary
Important 2-Mark Areas
- Define brucellosis.
- Define Legionnaires' disease.
- What is ESBL-producing bacteria?
- What is a capsule?
- What is swarming phenomenon?
- Define enteric fever.
- What is cholera?
- What is botulism?
- What is tetanus?
- What is gas gangrene?
Important 5-Mark Areas
- Virulence factors of Staphylococcus aureus
- Laboratory diagnosis of Staphylococcus aureus
- Virulence factors of Helicobacter pylori
- Laboratory diagnosis of H. pylori
- Laboratory diagnosis of gonorrhoea
- Virulence factors of Pseudomonas aeruginosa
- Laboratory diagnosis of enteric fever
- Virulence factors of Vibrio cholerae
- Diarrheagenic E. coli
- Laboratory diagnosis of Klebsiella
- Swarming phenomenon of Proteus
- Laboratory diagnosis of Haemophilus influenzae
- Virulence factors of Bacillus anthracis
- Gas gangrene
Important 10-Mark Areas
- Staphylococcus aureus — complete bacteriology
- Streptococcus — medically important species
- Neisseria — N. gonorrhoeae and N. meningitidis
- Helicobacter pylori — pathogenesis and laboratory diagnosis
- Pseudomonas aeruginosa — complete bacteriology
- Salmonella — pathogenesis and laboratory diagnosis
- Shigella — pathogenesis and laboratory diagnosis
- Vibrio cholerae — cholera
- Pathogenic Escherichia coli
- Klebsiella pneumoniae
- Bacillus anthracis
- Medically important Clostridium species