Lecture 5: Anaerobic Bacterial Infections

Clinical Signs of Anaerobic Bacterial Infections

Foul-smelling discharge (caused by short-chain fatty acid products of anaerobic metabolism)

Gas in soft tissue (production of CO2 and H2 gases)

Infection in proximity to a mucosal surface

Negative aerobic culture results

Gas-liquid chromatography (GLC) - Definitive identification

Clostridium

Gram-positive rods

Naturally found in soil, marine sediments, sewage

Most species are motile with peritrichous flagella (except C. perfringens, which lacks flagella)

Endospore-forming

Botulism

Clostridium botulinum

Produces neurotoxins Types A-G

A, B, E, F infect humans

Intoxication from contaminated food

Results in loss of muscle contraction → Flaccid paralysis

Visual disturbances – double vision

Difficulty swallowing

Speech difficulty

Respiratory paralysis, cardiac arrest → death

BSM (Botulism Selective Media)

Antitoxin

Tetnus

Clostridium tetani

Produces a neurotoxin - Tetanospasmin

Causes spastic paralysis

Tonic contraction of voluntary muscles

Lockjaw – mouth cannot be opened

Death occurs from interference with respiratory mechanisms

Tetanus toxoid vaccine (Tdap/Dtap vaccine)

Gas Gangrene

Clostridium perfringens

Non-motile

Produces:

Alpha toxin

Enterotoxin

Gas Gangrene Pathogenesis

Direct introduction of anaerobic cells or spores into wound

Spores germinate in the anaerobic environment of the dead/ischemic tissue

Alpha toxin:

Breaks down lecithin – a major component of cell membranes

Aggregates platelets, blocks blood small blood vessels, slows down blood (and oxygen) supply to tissues

Kill neighboring cells and the bacteria spread into dead tissue

Produce hydrogen and carbon dioxide as a result of tissue damage → frothy brownish fluid

Mortality

Gas Gangrene Clinical findings

Blackening of the infected muscle and skin

Intense pain at the initial site of infection

Secretion of a frothy brownish fluid resulting from the production of bubbles of hydrogen and CO2 gas

Bubbles are felt under the skin by palpation

Shock, and kidney failure

CP ChromoSelect agar – C. perfringens green colonies

Surgical debridement/amputation

IV penicillin

Hyperbaric oxygen therapy

Actinomycosis

Actinomyces

Non-spore forming

Branched filamentous gram-positive bacteria

Colonies on Anaerobe blood agar resemble molar teeth

Endogenous infection – mouth commensal

More often affects immunocompromised individuals

Chronic granulomatous infection

Causes tissue destruction, pyogenic lesions, interconnecting sinus formation

4 clinical forms

Actinomycosis clinical forms

Cervicofacial – ‘Lumpy jaw’ – commonest form

Affects the cheek and the submaxillary region

Can affect head, neck, eye

Thoracic – affects lungs – mild fever, cough, and purulent sputum

Abdominal – Ruptured appendix, ulcer, hepatosplenic infection

Pelvic – Colonization of intrauterine devices and subsequent invasion

Smelly pus caused by sulfur granules

Prolonged penicillin treatment

Clindamycin or erythromycin for patients with penicillin allergy

Surgical removal of affected tissue

Acne

Cutibacterium

Gram-positive, anaerobic rod, normal skin flora

Colonizes skin, especially sebaceous glands

Cutibacterium typically grows on sebum within the sebaceous glands

Excessive sebum secretion triggered by the hormones of adolescence stimulates bacterial growth

Increased number of bacteria attract leukocytes to the area

Leukocytes phagocytose bacteria and cause inflammation

Pus is formed by dead bacteria and leukocytes

For severe cases – antibiotics-doxycycline

Topical benzoyl peroxide

Bacteroides

Gram-negative rods

Intestine

Intra-abdominal infections

Usually follows surgical or traumatic damage of the intestinal wall, ruptured appendix

Abdominal pain, diarrhea, nausea, metabolic acidosis

Anaerobic bacteremia

Pelvic inflammatory disease, ovarian abscesses, arthritis

Capsular polysaccharides induces abscess formation

LPS is much less toxic

Enzymes:

Proteases, neuraminidases

Hemolysins: hemolysis of erythrocytes

Enterotoxin: diarrhea

Produces superoxide dismutase (SOD) and can survive in presence of oxygen

Bacteroides Bile Esculin (BBE) agar – Bacteroides growth makes the media black

Anaerobic culture on Brain Heart Infusion agar (BHIS)

Prevotella

Gram-negative rods

Upper respiratory tract infections

Female genital tract infections

CNS infections

Pelvic inflammatory disease

Ovarian abscesses

Anaerobic blood agar – tan to brown colonies

Porphyromonas

Gram-negative rods

Oropharyngeal and gingival infections

Breast, axillary, perianal infections

Male genital infections

P. GING agar

Fusobacterium

Pleomorphic, long gram-negative rods

Mouth & intestinal tract

Endotoxin producers

Respiratory infections

Complicating periodontal disease

Lemierre’s syndrome: Infectious thrombophlebitis of the internal jugular vein; potentially lethal

Associated w/ colon cancer & ulcerative colitis

FSA agar

Gardnerella vaginalis

Gram-variable bacilli – Very thin cell wall, does not retain crystal violet

Isolated from the female genitourinary tract

Causes bacterial vaginosis – inflammatory cells are not present

Vaginal discharge has a “fishy” odor

May have a vaginal itch, burning during urination

pH of the vaginal secretions is greater than 4.5 (normal pH is < 4.5)

Diagnosis: wet mount reveals “Clue cells” – vaginal epithelial cells covered with bacilli

Oral metronidazole