Medical Bacteriology II - Gram-Positive Bacilli

Lecture 7: Medical Bacteriology (II)

•  Lecture topic: Medical Bacteriology, focusing on bacilli bacteria, spirochetes, mycobacteria, and normal flora.
• Lecturer: Dr. Rasha Mokhtar Elnagar, specialist in medical microbiology and immunology.
•  The lecture covers Gram-positive and Gram-negative bacilli, including important genera and species.

Gram Positive Bacilli

•  Subdivided into spore formers and non-spore formers.
•  Spore formers include Bacillus anthracis and Bacillus cereus (aerobes) and C. perfringens, C. tetani, C. botulinum, and C. difficile (anaerobes).
•  Non-spore formers include Corynebacterium and Lactobacillus.
•  Bacillus anthracis forms a malignant pustule when spores germinate in skin.
•  Bacillus cereus causes food poisoning, due to enterotoxins.

Genus Bacillus: Spore formers

•  Morphology: gram-positive, spore-forming, aerobic or facultative anaerobic.
•  Important species: Bacillus anthracis, a zoonotic disease.
•  Important species: Bacillus cereus, causing food poisoning due to enterotoxins.

Genus Clostridium: Spore formers

•  Morphology: gram-positive, spore-forming, obligate anaerobes (only in the absence of oxygen).
•   All species are exotoxin producers.
•  Important species: Clostridium tetani (motile), causing tetanus (spastic muscle paralysis).
•  Important species: Clostridium botulinum (motile), causing botulism (flaccid muscle paralysis).
•  Important species: Clostridium perfringens (non-motile), causing gas gangrene.
•  Important species: Clostridium difficile, causing antibiotic-associated diarrhea and pseudomembranous colitis, due to long-term antibiotic therapy.
•  Culture conditions: blood agar in anaerobic jars or anaerobic incubators, and cooked meat media.

Genus Corynebacterium: Non-Spore formers

•  Morphology: Gram-positive bacilli with club-shaped ends, arranged in Chinese-letter arrangement.
•  Important species: Corynebacterium diphtheriae, causing diphtheria (upper respiratory tract infection).
•  Corynebacterium diphtheriae produces exotoxins.
•  Grow on Löffler's medium, in aerobic conditions, at 37°C.

2. Gram Negative Bacilli

•  Subdivided into rod-shaped, curved, and coccobacilli.
•  Important genera like Enterobacteriaceae, Vibrio cholera (Comma-shaped), H. pylori, Campylobacter, Brucella, Haemophilus.

1- Enterobacteriaceae

•  Gram-negative bacilli.
•  Facultative anaerobes
• E. coli, Klebsiella, Salmonella, Shigella, & Proteus: causes urinary tract infections, neonatal meningitis, pneumonia, bacteremia, enteric fever, food poisoning, and bacillary dysentery.
• Enterobacteriaceae are lactose fermenters as well as non-lactose fermenters. They also include motile and non-motile forms.

Curved Gram-negative bacilli

•  Vibrio cholerae: comma-shaped, motile bacilli, grows in alkaline conditions, like alkaline peptone water. Causes cholera (rice water diarrhea).
•  Helicobacter pylori: curved, motile bacilli. Grows in microphilic and acidophilic conditions. Causes peptic ulcers.

3-Coccobacilli

•  Includes genera like Brucella and Haemophilus influenzae.
•  Brucella (gram-negative coccobacilli).
• Causes zoonotic diseases: Malta fever (undulating fever).
• Important species: Brucella melitensis (goats and sheep) and Brucella abortus (cows and cattle).
• Haemophilus influenzae: causes meningitis and pneumonia in infants, grown on chocolate agar and needs 2 growth factors: X= hemin and V= NAD.

Pseudomonas aeruginosa

•  Gram-negative rod (pleomorphic), motile, obligate aerobic, causes urinary tract infections, bacteremia, bone and joint infections, particularly in patients with severe burns and immunodeficiency.

Spirochetes: Treponema pallidum

•  Spiral-shaped, motile (endoflagella).
•  Difficult to see with Gram stain.
•  Can be stained by Giemsa and silver stain.
•  Best viewed using dark field microscopy shows motility.
•  Cannot be cultured.
• Causes syphilis (sexually transmitted disease).

Genus Mycobacterium

•  Acid-fast, non- motile, non-spore forming bacilli.
•  Cannot be stained by Gram stain due to waxy lipid (mycolic acid) in the cell wall.
•  Important species: Mycobacterium tuberculosis (pulmonary tuberculosis) causes chronic cough, night fever, and sweating.
•  Important species: Mycobacterium leprae (leprosy).

Normal Flora (Commensal Bacteria)

•  Inhabits the human body.
• Mostly bacteria but may include fungi.
•  Usually does not cause disease in healthy individuals.
•  Beneficial roles: compete for nutrients and sites, produce antimicrobial substances, crucial for immune system of newborns, provide nutrients, maintaining acidic pH in the vagina.
•  Harmful effects: opportunistic infections in immunocompromised individuals. Disruption of normal flora due to antibiotics can lead to pathogenic overgrowth (e.g., Clostridium difficile) in the colon or Candida in the vagina causing infections.

Disease Distribution

•  Each site of the body has its typical group of normal flora.

Source and Mode of Transmission

• Pathogens can originate from the environment (soil, air, and water), animals (zoonotic diseases), or humans (cases and carriers).
• Transmission can take many routes (direct contact, indirect contact, airborne, droplet, ingestion, bloodborne, arthropod-borne, vertical, via placenta).

Case Study Summary

•  A 25-year-old woman presents with dysuria, mild pelvic tenderness, pus in urine, and Gram-negative bacilli in the urine.
•  This strongly suggests Escherichia coli as the cause of urinary tract infection