Infectious diseases notes summarising all infectious pathologies for medical school examinations. Note there is also a slight excess in specialist knowledge for medical school examinations.
Look at specialty section and content list for the summary contents of this file.
Staphylococci
Description Pathology
- Commensal: Skin / nasal commensal in ~80 % - S. aureus
- Coagulase: +ve organisms are more severe infx. o Toxin: Scald Skin Syn. (Bullae + desquam.)
- NB: S. epidermidis commonest CVC infx isolate o Food Toxin: Sudden D&V within hours
Metabolic o Toxic Shock: ↑TO; ↓GCS; Rash; V&D; Tampon
- Gram Stain: Positive o Soft Tissue: Impetigo; Cellulitis; Mastitis
- Aerobic: Facultative Anaerobes o Bony Tissue: Septic arthritis; Osteomyelitis
- Enzymes: Catalase ± Coagulase - S. saprophyticus
Coagulase o GUT: UTI ➔ Cystitis
- +Ve: S. aureus - S. gallinarum
- -Ve: S. epidermis; S. saprophyticus; S. gallinarum o Eye: Endophthalmitis (rare)
Diagnosis Management
- MC&S: Take sample from site of infection - MSSA: Flucloxacillin
- Novobiocin: Can differentiate - MRSA: Glycopeptide; 4 % CHG + Nasal Mupirocin
- NB: epidermidis (sensitive); saprophyticus (resistant) - VRSA: Linezolid; 4 % CHG + Nasal Mupirocin
Enterococcus Faecalis (Formerly Group D Streptococcus)
Description Management
- Commensal: Gut commensals - Key. E. Faecalis is sensitive to ampicillin but E.
- NB: Enterococcus formerly known as Group D Strep. faecium is resistant
Pathology - Endocarditis: Amoxicillin/Benzylpenicillin + Gent.
- UTI: Over 30 % of all bacterial UTIs - Meningitis: Intrathecal vancomycin (NB: VRE exist)
- Endocarditis: Assume if detected in blood - UTI: Nitrofurantoin (regardless of VRE)
- Meningitis: Rare neurosurgical complication - AMR: Resistant to cephalosporins and quinolones
Streptococci
Description Pathology
- α-Haemolytic: Haemoglobin ➔ Biliverdin (green) - S. pneumoniae
- β-Haemolytic: Haemoglobin ➔ Complete haemolysis o Lungs: Pneumonia
- Grouping: Lancefield group of β-Haemolytic o ENT: Otitis media
Species - S. pyogenes
- Non-Haemolytic/Non-typable Lancefield: S. viridans o Systemic: Scarlet fever; Peripartum sepsis
- α-Haemolytic: S. pneumoniae; S. viridans o ENT: Tonsillitis; Pharyngitis;
- β-Haemolytic group A: S. Pyogenes o Skin: Impetigo; Erysipelas; Cellulitis; NF
- β-Haemolytic group B: S. agalactiae o Complications: RHD; GN; Toxic shock
- β-Haemolytic group D: S. gallolyticus; S. bovis - S. viridians
- β-Haemolytic group F: S. milleri complex o Heart: Endocarditis (from dental caries)
Management - S. gallolyticus
- Pneumonia: Amoxicillin o Heart: IE
- RHD: Penicillin V o Risks: Colon/Liver disease; CRC
- Skin: Fucloxacillin - S. milleri
- Endocarditis: Benzylpenicillin + Gentamicin o Abscesses: Lung; Liver; Bartholin’s
,Clostridia
Description Pathology
- Spores: Resilient Spores - C. difficile
- Aerobic: Obligate anaerobe o GI: Bowel infection – See relevant notes
- NB: C. botulinum can tolerate low levels of O2 - C. botulinum
Species o Nerve: Botulism – See relevant notes
- Gut: C. difficile - C. tetani
- Skin: C. perfringens (formerly C. welchii) o Nerve: Tetanus – See relevant notes
- Nerve: C. botulinum; C. tetani - C. perfringens
Management o Muscle/Skin: Gas/Fournier’s gangrene (α-toxin)
- Basic: Early recognition and surgical debridement o Sx: Crepitations heard over gangrene
- C. Diff: Metronidazole; Vancomycin; Fidaxomicin o Risks: GI surgery; Open fracture; malignancy
Diphtheria
Description Investigation
- Location: E. Europe; Russia; Asia - MC&S: Culture/PCR
Species - Serology: Toxin detection
- Species: Corynebacterium diphtheriae Management
Pathology - Key: Airway control
- Emergency: Tonsillar (grey) pseudomembrane - Antitoxin: Give within 48 hrs
- Sx: Fever; Odynophagia; Cervical lymphadenopathy - Abx: IM Benzylpenicillin / Erythromycin
Actinomycosis
Species Pathology
- Species: Actinomyces israelii - Subacute: Granulomatous/suppurative infx
Management - Location: Occurs adjacent to mucous membrane
- Abx: Penicillin - NB: Sulfur granules in pus/tissue characteristic
Neisseria meningitidis
Description Investigations
- Commensal: URT in 10 %; 25 % of teens - Microscopy: CSF; Blood; Skin lesion biopsy
- NB: Adheres to non-ciliated epithelial cells - NB: Intra/Extra-cellular diplococci
- Transmission: Aerosolized droplets Management
- Virulence: Serotypes ABCWY are virulent - Meningitis: Dexamethasone
- Microscopy: Gram negative diplococci - Prevention: Group B vaccination; ACWY at 14 YO
- Risks: Age; Complement defects; HIV; ↓Spleen - Abx: Ceftriaxone; Benzylpenicillin
Pathology - Prophylaxis: Ciprofloxacin/Ceftriaxone one dose
- Meningitis (50 % of cases) - Alternative: Rifampicin BD for 48 hrs
o Initial Sx: Headache; N&V; Malaise Complications
o Late Sx: Neck rigidity; Photophobia; ↓ GCS - Meningitis: ≥25 %; SNHL; Epilepsy
- Meningococcaemia - Systemic: AKI; ARDS; Thrombosis; Pneumonia
o Mild Sx: Fever; Macular rash; No shock signs
o Severe Sx: ↑To; Septic shock; Coagulopathy
Moraxella
Description Pathology
- Commensal: URT coloniser in children (↓ in adults) - Lungs: Bronchitis; Sinusitis; Laryngitis
- Risks: COPD; Elderly - ENT: Otitis media (esp. in children)
Species - Systemic: Septicaemia ➔ IE (20 % mortality)
- Species: Moraxella catarrhalis - MSK: Septic arthritis (rare)
Investigations Management
- MC&S: Sputum; Ear effusion; Sinus aspirate; Blood - Abx: Macrolide; Cephalosporin
- NB: Hockey puck sign (colonies move around on agar)
Escherichia coli
Description Pathology
- Commensal: Part of normal colonic flora - Enterotoxigenic: Traveller’s diarrhoea
- Pathogens: Many different pathogenic forms - Enterohaemorrhagic: D + haemorrhagic colitis
- Character: Facultative anaerobic; Lactose fermenter - Enteropathogenic: Esp. infants in poor sanitation
Management - Enteroinvasive: Dysentery-like syndrome
- Abx: TMP; Ampicillin; Cephalosporin; Ciprofloxacin - Enteroadherent: Traveller’s diarrhoea; Chronic D
- NB: ESBL and CRE commoner in E. coli strains - Extra-intestinal disease: Neo. sepsis; Pneumonia
Klebsiella pneumoniae
Description Pathology
- Commensal: Skin; Nasopharynx - Lungs: Pneumonia (alcoholics; Dysphagia pt.)
- Risks: Abx exposure; Catheter; ↓ Immunity - ENT: Nasopharyngitis
- NB: Very contagious in prolonged close quarters - GUT: UTI
Management - Complications: Sepsis + Necrosis if ↓ Immunity
- Abx: Aminoglycoside; Cephalosporin; Carbapenem
- Note: ESBL and CRE commoner in K. pneumoniae
Proteus mirabilis
Description Pathology
- Stones: Colonise stones formed in GUT - GUT: UTI (see Nephrolithiasis notes in urology)
- Metabolic: Urease which ↑ risk of stone formation - Risks: Struvite; Calcium oxalate; Uric acid stones
- Sex: More common in men - Complications: Renal calculi ➔ AKI
Presentation Management
- Signs: Alkaline urine; Fishy odour - Abx: Gentamicin; Carbapenems; Cephalosporins
Investigations - Resistant: Tetracycline; Nitrofurantoin
- Culture: Urine culture - NB: 10-20 % are resistant to ampicillin
- Microscopy: Characteristic swarming motility
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