Download Staphylococcus

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
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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
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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