GRAM POSITIVE COCCISTAPHYLOCOCCI

BY OYOM P. ANTHONY

2013-BMLS-FT-009

LINNAEAN

CLASSIFICATION Kingdom:Bacteria

Phylum: Firmicutes

Class: Bacilli

Order: Bacillales

Family: Staphylococcaceae

Genera: Staphylococcus

Species:Staphylococcus Aureus

Staphylococcus epidermidis

Staphylococcus capitis

Staphylococcus saprophyticus

Staphylococcus hominis

STAPHYLOCOCCI

Gram positive aerobic organisms Reproduce asexually by binary fission Common microorganism in the

environment; present in air, water and dust

Common strains are S. aureus, S. epidermidis and S. saprophyticus

S. aureus commonly inhabits nasal passages and axillae.

S. epidermidis is a normal flora on the skin.

S. saprophyticus rare but may inhabit the female genital tract

MORPHOLOGY AND STAINING

Are gram positive

Spherical, form clusters due to

division in 3 planes, after which the

bacteria remain attached to each

other.

MORPHOLOGY AND STAINING

Are non-motile

Non spore forming

Are non-capsulated except in young

cultures; capsulation is lost with

prolonged culturing

CULTURE

CHARACTERISTICS Grow aerobically and are facultative

anaerobes.

Capable of growing at temperature s between 220C – 440C (Ideal temp 350C -370C)

Grows on ordinary media; nutrient agar & blood agar

Colonies are 1 – 3 mm diameter, smooth, low convex, glittering and opaque on nutrient agar.

S. aureus will yield large yellow low convex colonies with β-haemolysis (esp. fresh isolates) on blood agar

S. epidermidis will yield small creamy/ white colonies with no haemolysis on blood agar

CULTURE

CHARACTERISTICS Pigmentation in S. aureus may be

enhanced by: aerobic incubation, use

of fatty media (e.g. tween agar), or

prolonged incubation of the plate.

MacConkey Agar will yield small pink

colonies 0.5 – 1mm diameter (for

lactose fermenting strains)

Modified MacConkey Agar yields no

growth due to inhibition by crystal

violet

CULTURE

CHARACTERISTICSStaphylococcus species are salt

tolerant, will grow in selective media

such as:

Cooked meat broth with 10% NaCl –

enrichment of S. aureus

Milk Agar with 7 – 10 % NaCl – for

primary plating and pigmented

colonies

Mannitol Salt Agar – for isolation of S.

aureus

PATHOGENESIS

Staphylococcal infections are

common in hospitals & communities.

S. aureus is the most pathogenic, but

S. epidermidis increasingly associated

with nosocomial infections.

S. saprophyticus associated with

urinary tract infections, especially in

sexually active young females.

PATHOGENESIS: Risk

Factors Neonates and breastfeeding mothers Chronic skin disorders Patients on immunosuppressants Chronic broncho-pulmonary disorders

e.g. emphysema Patients with implants or prosthetics Patients with indwelling catheters Patients with surgical incisions Patients with diabetes mellitus Patients with burns

PATHOGENESIS

Diseases resulting from tissue invasion include:

Skin infections (cutaneous abscesses, mastitis, wound and burn infections)

Neonatal infections (pneumonia, meningitis, skin lesions)

Pneumonia (Not common in community setting)

Endocarditis (particularly in IV drug users and patients with prosthetic heart valves)

Osteomyelitis (especially in children)

PATHOGENESIS

Toxin mediated diseases include:

Toxic Shock Syndrome (via vaginal

tampons, wound and burn infections)

Scalded skin syndrome (common in

infants)

Staphylococcal Food Poisoning

(commonly from canned foods,

pastries and salads)

PATHOGENESIS: Virulence

Factors Protein A: binds to IgG, inhibits phagocytosis

Leukocidins: specifically acts against PMN leucocytes, damages membranes

Catalase: neutralizes some super-oxides in phagosomes

Coagulase: causes localized clotting

Haemolysins: causes destruction of erythrocytes

Exotoxins: e.g. TSST-1, Enterotoxin, Exfolatins

Resistance to antimicrobial agents (inherent or acquired)

Carotenoids (Antioxidant, resists action of super-oxides)

Biofilms: especially S. epidermidis, resist phagocytosis

DIAGNOSIS

Samples collected for diagnosis will depend on type of staphylococcal infection e.g. ;

Scalded skin syndrome – from abnormal skin, blood or urine

Food poisoning – stool or suspect food

Osteomyelitis – bone biopsy (since X-ray might not show changes for 10-14 days after infection)

Endocarditis – blood for culture

Other specimens include pus, sputum

DIAGNOSTIC TESTS

Gram Stain: Will reveal purple cocci

occurring in clusters.

Culture: MacConkey Agar, Blood

Agar, Mannitol Salt Agar can be used

Biochemical Tests: Catalase,

Coagulase, DNAse

NOTE: Susceptibility testing

recommended due to increased

incidence of MRSA.

DIAGNOSTIC TESTS:

CULTURE Blood Agar: S. aureus forms large

cream or yellow low convex colonies with β-haemolysis, S. epidermidis forms

small white colonies with no

haemolysis.

MacConkey Agar: small pale pink

colonies observed.

Mannitol Salt Agar: Large yellow

colonies

S. aureus, S. epidermidis (Blood

Agar)

DIAGNOSTIC TESTS:

BIOCHEMICAL TESTS Catalase Test: Presence of effervescence

will indicate presence of staphylococcus, rule out streptococcus.

Coagulase: coagulation observed will indicate presence of pathogenic strain (S. aureus)

DNAse: zone of clearance observed will indicate S. aureus

Novobiocin test: growth <12mm or uniform growth till edge of disk will indicate resistance (S. saprophyticus) & clearance zone >16mm will indicate susceptibility (S. epidermidis)

Catalase & Coagulase Tests

Mannitol Salt Agar, DNAse

DNAse, Novobiocin

MANAGEMENT/TREATMEN

T Drainage of abscesses

Removal of catheters

Fluid replacement and electrolyte

balancing

Patients with MRSA should be

isolated

Administration of antimicrobials

TREATMENT

Choice and dosage of antibiotics depend on:

Infection site Illness severity Probability that resistant strains are

presentMany strains produce β-lactamase &

therefore resist Penicillin G & V.These can be treated with Methicillin,

Nafcillin or oxacillins.MRSA strains are resistant to the above,

can be treated with Vancomysin.VRSA strains have also emerged,

however.

PREVENTION AND

CONTROL Thoroughly sterilize reusable equipment;

organism is vulnerable to alcohol based sterilizers and moist heat at 600C.

Disinfection of hands between patient examinations

Isolation of MRSA patients

Maintenance of proper sanitation and body hygiene when handling food

Immediately cleaning and treating skin scratches, abrasions or puncture wounds

Use of protective gear when handling patients

EPIDEMIOLOGY

Staphylococcus, especially S. aureus is a major cause of nosocomial and community acquired infections.

Humans are a natural reservoir for S. aureusand S. epidermidis.

Young children will have higher colonization rates due to frequent contact with respiratory secretions & other exposures.

Spread is rapid in crowded areas with poor sanitation.

MRSA, originally associated with hospitals is increasingly acquired in communities.

THANKS FOR YOUR

ATTENTION