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

• 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 strepsS. aureus

Haemophilus sppAnaerobes

And many more

Angular cheilitis: multifactorial including …

Candida spp, S. aureus, beta haemolytic streps

Staphylococcal mucositis

Local staphlococcal infectionsinside 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