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Staphylococcus Dr Julian Ng General • About 40 known Staphylococcus spp. • Gram Stain: Gram positive coccus; 0.5µm1.5µm • usu. arranged in grape-like clusters but may also be seen as pairs/tetrads or short chain • All except S. saccharolyticus and S. aureus subsp. anaerobius are facultative anaerobes • Grows readily in most culture media and can grow in the presence of 10% NaCl • Generally, they are catalase positive (rare exceptions) Clinical Significance • Most are opportunists • Can colonize skin and mucous membranes • Breaks in the epithelial barrier may allow them to becomes pathogenic S. aureus • • • • Clinically most important species Can cause a wide variety of human diseases Possess many virulence factors Up to 35% of humans are persistent nasal carriers • Easily transferrable from human to human via skin contact – Importance in infection control esp. in Methicillinresistant Staphylococcus aureus (MRSA) • Most common cause of nosocomial pneumonia and skin and soft tissue infections • 2nd most common staphylococcal spp. to cause primary bacteraemia in hospitals • Typical colony: Pigmented (cream yellow to orange), haemolytic on blood agars • Biochemical characteristics: Catalase positive, Coagulase positive, slide agglutination (clumping factor) positive Key Test Clinical spectrum • Any localised infection may become invasive and can lead to bacteraemia • Systemic infections such as primary or secondary bacteraemia, endocarditis, meningitis can occur • Toxin-mediated diseases includes staphylococcal toxic shock syndrome, staphylococcal food poisoning, staphylococcal scaled skin syndrome Localised infections • Very common cause of infection by staphylooccal spp. • Often results in pus formation • Can result in skin, soft tissue infection or deep abscesses Impetigo Boil (Furuncle) Carbuncle Stye Surgical wound infections: many causes including S. aureus Oral infections • • • • Acute parotitis Angular cheilitis Mucositis Etc Acute parotitis: various causes Including bacteria … Alpha-haemolytic streps S. aureus Haemophilus spp Anaerobes And many more Angular cheilitis: multifactorial including … Candida spp, S. aureus, beta haemolytic streps Staphylococcal mucositis Local staphlococcal infections inside oral cavity Toxin-mediated • Toxic shock syndrome toxin (TSST-1) is a super-antigen capable of activating large number of T cells • Was associated with use of tampons but is also known to be associated with postoperative wound or soft tissue infections • Preformed, heat-resistant enterotoxin mediates staphylococcal food poisoning (symptoms in 2-6 hours; usu self-limiting) • Exfoliative toxins A and B results in staphylococcal scalded skin syndrome; usu in infants and neonates • Panton-Valentine Leukocidin (PVL) consists of 2 components S and F, together with γ exotoxin lyses WBC resulting in massive release of inflammatory mediators responsible for necrosis and severe inflamation • PVL is an important virulence factor in MRSA infections MRSA • Methicillin-resistant S. aureus • Resistant to all penicillins, cephalosporins, and penems • Usually multiply-resistant • Vancomycin resistance is very rare – so far • Hospital-acquired • Community-acquired cases now (CA MRSA) Coagulase-negative staphylococcal spp (CoNS) • S. epidermidis – most frequently isolated staphylococcal spp. • Colonises moist body areas such as auxillae, inguinal and perianal areas, anterior nares and toe webs • Important cause of nosocomial infection esp. S. epidermidis • Usu causes nosocomial infections in patients with predisposing factors such as immunodeficiency/ immunocompromised or presence of foreign bodies • Ability to form biofilm is most important factor in foreign body infections by CoNS – Important to remove/ replace foreign body in treatment • S. saprophyticus frequently isolated in rectum and genitourinary tract of young women • Can be causative agent in UTI in young healthy women • 2nd most common urinary pathogen (other than E. coli) in uncomplicated cystitis in young women • Colony counts of ≥ 105 CFU/ml usu. indicative of significant bacteriuria Line-related sepsis • Frequently staphylococcal • CNS common • S. aureus particularly serious Line-related sepsis with S. aureus = get help from Infectious Disease physician Antimicrobial susceptibility • MRSA can be due to 3 different resistance mechanisms – Production of penicillin-binding protein 2a (PBP2a) encoded by mecA gene – Production of beta-lactamase – Production of modified intrinsic PBPs • Resistance due to mecA can be detected via cefoxitin disk diffusion or dilution methods according to CLSI breakpoints (≤ 21mm – resistant, ≥ 8µg/ml – resistant, respectively) • Resistance due to beta-lactamase production can be detected via the use of beta-lactamase inhibitor such as clavulanic acid which would result in an increase in zone size (disk diffusion method) or decrease of 2 dilutions • Vancomycin-intermediate S. aureus (VISA) is thought to be due to changes in cell wall • S. aureus with vancomycin minimum inhibitory concentration (MIC) of 4-8µg/ml are VISA according to CLSI guidelines • VRSA due to acquisition of vanA gene was first reported in 2002 in US • Vancomycin MIC ≥ 16µg/ml = VRSA VRSA uncommon Treatment • Drain pus, remove foreign material and dead tissue • Methicillin – cloxacillin • (Erythromycin, clindamycin) • Vancomycin • Topical agents: e.g. mupirocin References • Manual of Clinical Microbiology 10th Ed. Chap 19 pp 308-330 • Jawetz, Melnick, Adelberg’s Medical Microbilogy 25th Ed. Chap 13 pp 185-190