Microbiology: Gram-Negative Bacteria Infections

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