Medical Microbiology: Key Bacteria, Diseases and Diagnostics
Medically Important Bacteria: Pathogens, Diagnosis and Treatment
Gram-Positive Cocci
Streptococcus pyogenes: Gram-positive, catalase-negative cocci. Causes pharyngitis, impetigo, scarlet fever, erysipelas and streptococcal toxic shock. Diagnosis: bacitracin sensitivity and the antistreptolysin O (ASO) or anti-DNase B tests. Treatment: penicillin.
Streptococcus pneumoniae: Gram-positive, lancet-shaped diplococci with a polysaccharide capsule. Causes pneumonia, sinusitis and meningitis. Diagnosis: blood agar (alpha-hemolysis), optochin sensitivity and bile solubility. Treatment: penicillin.
Staphylococcus aureus: Gram-positive, catalase-positive, facultatively anaerobic cocci. Causes food poisoning (enterotoxin), osteomyelitis, sepsis and necrotizing infections. Diagnosis: coagulase test and mannitol salt agar. Treatment: cloxacillin.
Gram-Positive Bacilli
Mycobacterium tuberculosis: Acid-fast, strictly aerobic bacillus with a cord factor and a high mycolic acid content. Causes tuberculosis. Diagnosis: Ziehl-Neelsen stain, Lowenstein-Jensen medium, niacin test and tuberculin (Mantoux) test. Treatment: multidrug regimen (isoniazid, rifampicin, ethambutol, pyrazinamide).
Mycobacterium leprae: Acid-fast bacillus that cannot be cultured on artificial media. Causes leprosy: lepromatous form (negative lepromin test) and tuberculoid form (positive lepromin test). Diagnosis: Fernandez-Mitsuda test and armadillo infection model. Treatment: dapsone, rifampicin and clofazimine (multidrug therapy); BCG vaccine.
Clostridium and Bacillus anthracis: Spore-forming, large Gram-positive rods. Bacillus anthracis is capsulated and causes anthrax; Bacillus cereus causes food poisoning.
Gram-Negative Cocci
Neisseria gonorrhoeae: Gram-negative diplococcus, aerobic, oxidase-positive. Causes urethritis, cervicitis and ophthalmia neonatorum. Diagnosis: Gram stain and Thayer-Martin medium. Treatment: ceftriaxone and azithromycin.
Neisseria meningitidis: Gram-negative diplococcus with a polysaccharide capsule. Causes meningitis and sepsis. Diagnosis: CSF analysis (low glucose), Thayer-Martin medium. Prophylaxis: rifampicin or ciprofloxacin; meningococcal vaccine.
Gram-Negative Bacilli
Salmonella typhi: Motile, flagellated, lactose-negative Gram-negative bacillus. Causes typhoid fever and gastroenteritis. Diagnosis: blood culture (first week), stool and urine culture (second week), Widal test. Treatment: ciprofloxacin or ceftriaxone.
Shigella: Non-motile Gram-negative bacillus. Causes bacillary dysentery. Diagnosis: stool culture on SS agar. Treatment: ciprofloxacin, ampicillin.
Yersinia pestis: Gram-negative coccobacillus, capsulated, bipolar staining. Causes bubonic, pneumonic and septicemic plague. Diagnosis: blood and lymph node culture. Treatment: streptomycin or gentamicin, doxycycline.
Proteus: Motile, urease-positive, swarming Gram-negative bacillus. Often nosocomial. Treatment: cephalosporins.
Klebsiella pneumoniae: Capsulated Gram-negative bacillus. Causes lobar pneumonia, rhinoscleroma and urinary tract infections. Diagnosis: colony count of 100,000 CFU/mL. Treatment: third-generation cephalosporins.
Haemophilus influenzae: Small, pleomorphic Gram-negative coccobacillus. Type b causes meningitis, epiglottitis and pneumonia in children aged 3 months to 3 years. Diagnosis: chocolate agar, X and V factor requirement. Prevention: Hib vaccine (DTaP).
Chlamydia trachomatis: Obligate intracellular coccus. Causes trachoma, urethritis and lymphogranuloma venereum. Diagnosis: basophilic inclusions in cells and Frei test. Treatment: tetracyclines or azithromycin.
Treponema pallidum: Spirochete, microaerophilic or anaerobic. Causes syphilis: primary (painless hard chancre), secondary (condylomata lata, rash), tertiary (gummas, neurosyphilis). Diagnosis: VDRL, RPR, confirmation with FTA-ABS and TPHA. Treatment: penicillin G.
Anaerobic Spore-Forming Bacteria
Clostridium tetani: Motile, spore-forming, obligate anaerobic bacillus. Produces tetanospasmin, a neurotoxin that blocks glycine and GABA release, causing spasms and hyperreflexia. Diagnosis: blood agar. Prevention: tetanus antitoxin and DTaP vaccine.
Clostridium perfringens: Motile, spore-forming anaerobic bacillus. Produces lecithinase, collagenase and enterotoxin; causes gas gangrene and food poisoning. Treatment: hyperbaric oxygen, debridement and penicillin.
Clostridioides difficile: Anaerobic bacillus producing toxins A and B. Causes pseudomembranous colitis, often after antibiotic use. Treatment: vancomycin or metronidazole.
Clostridium botulinum: Motile, spore-forming anaerobic bacillus. Produces botulinum neurotoxin, which inhibits acetylcholine release and causes flaccid paralysis (botulism). Diagnosis: thioglycollate broth. Treatment: antitoxin and supportive care.
Other Important Bacteria
Brucella (B. suis, B. melitensis, B. abortus): Small, aerobic, non-motile Gram-negative coccobacilli. Cause brucellosis (undulant fever). Diagnosis: Brucella agar and the Huddleson test. Treatment: doxycycline with rifampicin.
Corynebacterium diphtheriae: Aerobic, non-motile, club-shaped Gram-positive bacillus with metachromatic granules and exotoxin production. Causes diphtheria (pharyngitis and peripheral neuropathy). Diagnosis: Loeffler medium and tellurite agar. Treatment: diphtheria antitoxin and penicillin or erythromycin.
Bordetella pertussis: Strictly aerobic, non-motile coccobacillus. Produces pertussis toxin, filamentous hemagglutinin and endotoxin; causes whooping cough. Diagnosis: Bordet-Gengou medium. Treatment: macrolides (azithromycin); prophylaxis with DTaP vaccine.
Pseudomonas aeruginosa: Strictly aerobic, motile Gram-negative bacillus producing pyocyanin, a blue-green pigment. Causes burn wound infections, urinary tract infections and pneumonia. Treatment: gentamicin, ciprofloxacin or ceftazidime.
Serratia marcescens: Facultatively anaerobic, motile Gram-negative bacillus with O and H antigens and red pigment production. Causes urinary, wound and bloodstream infections. Diagnosis: Voges-Proskauer test. Treatment: aminoglycosides or fluoroquinolones.