Microbiology: Gram-Negative Bacteria Infections

Study Notes

Haemophilus

Haemophilus influenzae (Respiratory)
- Found on the mucous membranes of the upper respiratory tract in humans
- Causes meningitis in unvaccinated children and upper and lower respiratory tract infections in children and adults
- Gram-negative, short coccobacilli in fresh medium, pleomorphic forms in old medium
- Requires certain growth factors called X and V, with Factor X acting physiologically as hemin and Factor V as nicotinamide adenine dinucleotide (NAD)
Antigenic Structure
- Serotyping: Polysaccharide capsule composed of polyribitol ribose phosphate (PRP) confers virulence, with 6 types of capsules (a, b, c, d, e, and f)
- Biotyping: based on the production of indole, ornithine decarboxylase, and urease
Diseases
- Otitis media in children
- Meningitis: in children between the age of 6 months to 3 years
- Acute epiglottitis: sore throat and fever, with severe upper airway wheezing (stridor) and inability to swallow
- Septic arthritis: the most common cause in infants
- Sepsis: in children between 6 months to 3 years
Vaccination (Hib capsule vaccine)
- Purified type b capsule, effective in generating antibodies in children older than 18 months
- Women in the eighth month of pregnancy, resulting in increased antibody secretion in breast milk (passive immunization)

Haemophilus ducreyi

Causes chancroid, a sexually transmitted disease (STD)
Causes a painful genital ulcer, painful swollen inguinal lymph nodes, and the lymph nodes become matted and rupture, releasing pus
Different from syphilis, which is painless and has no pus

Bordetella pertussis

Causes whooping cough
Gram-negative coccobacilli, capsulated, and strict aerobe
Requires Bordet-Gengou medium (potato-blood-glycerol agar) containing penicillin G
Four virulence factors:
- Filamentous hemagglutinin (FHA): a pili that binds to ciliated epithelial cells of the bronchi
- Pertussis toxin: exotoxins with B subunit (Bind) and A subunit (A for Action), overactivating cAMP, promoting lymphocytosis and sensitization to histamine
- Adenylate cyclase toxin (ACT): synthesizes messenger cAMP, impairing chemotaxis and generation of H2O2 and superoxide, and inhibiting phagocytosis and clearing bacteria
- Tracheal cytotoxin: destroys ciliated epithelial cells, resulting in impaired clearance of bacteria, mucus, and inflammatory exudate
Pathogenesis
- Highly contagious disease
- Transmission: via respiratory secretions on the hands or in an aerosolized form
- Incubation period: one week
- Three stages of the disease:
  - Catarrhal stage: lasts from 1-2 weeks, with low-grade fevers, runny nose, sneezing, and cough
  - Paroxysmal stage: lasts a month or longer, with attacks of nonproductive cough, vomiting after the attack, and hypoxemia and cyanosis
  - Convalescent stage: attacks become less frequent over a month, and the patient is no longer contagious
Diagnosis:
- Blood test: high white blood count (16,000–30,000/μL) with absolute lymphocytosis
- Specimens: Nasopharyngeal (NP) with Dacron swab
- Bordet-Gengou medium, PCR, and serology (ELISA)
Vaccination
- Combined with formalin-inactivated tetanus and diphtheria toxoids to form the DTaP (Diptheria-Tetanus- acellular Pertussis)
- Schedule: administration of doses at 2, 4, 6, and 15–18 months of age, with a booster dose at 4–6 years of age

Brucellae

Obligate parasites of animals and humans, intracellular
All Brucella species names are based on the animal they infect (e.g. Brucella melitensis from goats, Brucella abortus from cows)
Causes Brucellosis (undulant fever, Malta fever)
Transmission: direct contact with infected animal meat or aborted placentas, or ingestion of infected milk products
Pathogenesis and Pathology
- Portal of entry to bloodstream via lymphatic channels and regional lymph nodes
- Infected organs: Osteomyelitis, meningitis, or cholecystitis
- B. melitensis infection is more acute and severe than other species
Clinical Findings
- Incubation period: 1 to 4 weeks
- Malaise, fever, weakness, aches, and sweats, with fever rising in the afternoon and falling during the night
Diagnostic Laboratory Tests
- Specimens: blood and bone marrow
- Culture: Trypticase-soy medium with or without 5% sheep blood, Brain–heart infusion medium, and Chocolate agar
- Incubation: 8–10% CO2 at 35–37°C for 3 weeks
- Serology: IgM and IgG antibodies, with IgM rising during the first week and peaking at 3 months, and IgG rising during the first week and peaking at 6-8 weeks