Staphylococcus Dr Julian Ng

General About 40 known Staphylococcus spp. Gram Stain: Gram positive coccus; 0.5µm- 1.5µm 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 Methicillin- resistant Staphylococcus aureus (MRSA)

Most common cause of nosocomial pneumonia and skin and soft tissue infections 2 nd 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 2 nd most common urinary pathogen (other than E. coli) in uncomplicated cystitis in young women Colony counts of ≥ 10 5 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 10 th Ed. Chap 19 pp Jawetz, Melnick, Adelberg’s Medical Microbilogy 25 th Ed. Chap 13 pp