Gram Positive Cocci - Staphylococci

STAPHYLOCOCCI

Staphylococci - derived from Greek “staphyle” (bunch of grapes). They are gram positive cocci arranged in clusters. More than 20 species including: S. aureus, S. saprophyticus,S. epidermidis.

STAPHYLOCOCCUS AUREUS

PATHOGENICITY

SUPERFICIAL INFECTIONS

Localized skin infection - Folliculitis, furuncle (boil), carbuncle, abscess (particularly in breast), wound infection (surgical, traumatic), impetigo

DEEP INFECTIONS

Deep, localized infections – Osteomyelitis (S. aureus is the most common cause), arthritis (septic arthritis in children)

Respiratory tract infections - bronchopneumonia, tonsillitis, pharyngitis, sinusitis, lung abscess, empyema

CNS infections – abscess, meningitis, intracranial thrombophlebitis

Endovascular – bacteremia, septicemia, pyemia, endocarditis

Urinary tract infection often in association with local instrumentation, implants or diabetes

Nosocomial infections – wound infection, catheter associated bacteremia.

TOXIN MEDIATED INFECTIONS

Food poisoning

Occurs 1 to 6 hrs after consumption of food contaminated with preformed toxins (enterotoxins – very fast acting).

Nausea, vomiting, abdominal cramps & diarrhoea

Self limiting condition, lasting for 1 to 3 days.

Highest risk of contamination: sliced meat, puddings, pastries, sandwiches, milk & cheese

Toxic shock syndrome

Caused by Toxic shock syndrome toxin.

Was discovered in association with menstrual (tampon related TSS) cases.

Fatal multisystem disease occurring mostly in females.

Prodromal period of 2-3 days is followed by high grade fever and/or chills, Nausea and/or vomiting, profuse watery diarrhea with abdominal pain, mucosal hyperemia (vaginitis, conjunctivitis) & erythematous rash which desquamates in 1-2 weeks.

Also myalgias and/or arthralgias, Headache, Confusion occur. 

Scalded skin syndrome

Exfoliative skin disease – outer layer of epidermis gets separated from the underlying tissues.

Cause – exfoliative toxin that damages desmosomes

Severe forms of SSSS are: “Reiter’s Disease” in newborns and

       Toxic epidermal necrolysis in older patients

Milder forms are: Pemphigus neonatorum, Bullous impetigo

Signs And Symptoms: Fever, Generalized erythema, exfoliation or desquamation, painful skin.

Skin slips off with gentle pressure leaving wet red areas (Nikolsky sign)

LABORATORY DIAGNOSIS

SPECIMEN

1.     Pus - Suppurative lesion

2.     Sputum – respiratory tract infection

3.     Feces & remains of food – food poisoning

4.     Blood – septicemia

5.     Nasal swab – detection of carriers. Etc

DIRECT MICROSCOPY

Gram +ve spherical cocci in grape like clusters. May also be found in single, pairs and short chain forms. They are nonmotile and non sporing.

CULTURAL CHARACTERISTICS

Facultative Anaerobe. 37ºC and pH 7.4

Nutrient Agar: Smooth, convex, opaque, circular colonies on nutrient agar with golden yellow pigment formation.

Nutrient agar slope: the confluent growth presents a characteristic oil paint appearance.

Blood Agar: Beta haemolysis on blood agar. Haemolysis is marked on rabbit or sheep blood. 

Liquid Media: Uniform turbidity in liquid media.

Selective Media: Ludlam’s media, Mannitol salt agar (MSA), Salt milk agar.

Macconkey Agar: Small pink colonies due to lactose fermentation.

BIOCHEMICAL REACTIONS

Catalase: This test is performed using 3% H₂O_2. All Staphylococci are catalase positive, give prompt effervescence.

Coagulase Test: Done by two methods:

Slide coagulase test – detects bound coagulase (clumping factor)

Tube coagulase test – detects free coagulase

Mannitol: Generally staph aureus ferments mannitol but other species do not, so mannitol fermentation helps in distinguishing staph aureus from other staph species.

DISTINCTIVE DIAGNOSTIC FEATURES OF STAPHYLOCOCCUS AUREUS

1.     Coagulase Production	2.     Thermostable Nuclease Production
3.     Mannitol Fermentation	4.     Complete(beta) Haemolysis
5.     Phosphatase Production	6.     Golden yellow pigment
7.     Sensitive to lysostaphin	8.     Liquefy gelatin

ANTIBIOTIC SENSITIVITY TESTING

Media

Muller Hinton agar (MHA) (pH 7.2-7.4).

Method

Kirby-Bauer Disc Diffusion Method

Drugs

Penicillins, cephalosporins, gentamicin, erythromycin, clindamycin, tetracycline, cotrimoxazole, vancomycin etc.

Typing

PHAGE TYPING is done for epidemiological purposes

Other typing methods include antibiogram pattern, serotyping, plasmid profile, DNA fingerprinting, ribotyping and PCR based analysis for genetic pleomorphism.

COAGULASE NEGATIVE STAPHYLOCOCCI

Staphylococcus epidermidis:

Commensal on skin, common cause of stitch abscess. Can cause septicaemia because it has predilection to grow on implanted foreign body such as heart valves and shunts. It also causes endocarditis in drug addicts. It is sensitive to novobiocin.

Staphylococcus saprophyticus:

Commensal on skin and periurethral area. It causes urinary tract infections in young females. It is resistant to novobiocin.

HOSPITAL INFECTIONS BY STAPHYLOCOCCI

Hospital infections by staphylococci deserve special attention because of their frequency and because they are caused by strains resistant to various antibiotics. Staphylococci are a common cause of postoperative wound infection and other hospital cross infections. Most of these are due to certain strains of staphylococci that are present in the hospital environment, the so called ‘hospital strains’. Some of them, the ‘epidemic strains’, cause epidemics of hospital cross infections.

METHICILLIN RESISTANT STAPHYLOCOCCUS AUREUS (MRSA)

What is MRSA?

•        MRSA refers to the resistance of a strain of Staphylococcus aureus to the Beta lactam class of antibiotics, which includes methicillin, nafcillin, oxacillin, cloxacillin, dicloxacillin, imipenem, and cephalosporins. (May also be resistant to clindamycin, erythromycin and aminoglycosides.) MRSA, therefore, poses a treatment problem, since the usual antibiotics used to treat Staphylococcus aureus infections (cloxacillin) cannot be used. As a result more toxic and expensive antibiotics (eg. vancomycin) must be used instead.

Common sources and modes of transmission of MRSA

Two types of MRSA: Community acquired (CA-MRSA) and Hospital acquired (HA-MRSA).

• CA-MRSA occurs in otherwise healthy people who have not been recently (within the past year) hospitalized nor had a medical procedure (such as dialysis, surgery, catheters). CA-MRSA infections include skin infections like abscesses, boils, and other pus-filled lesions.
• HA-MRSA In hospitals, the most important reservoirs of MRSA are infected or colonized patients.

HOSPITAL PERSONNEL: commonly identified as a link for transmission between colonized or infected patients.

Transmission: via hands (especially health care workers') this may get contaminated by contact with:    1. Colonized or infected patients

             2. Colonized or infected body sites of the personnel themselves,

3. Devices, items, or environmental surfaces contaminated with body fluids                    containing MRSA.

Laboratory detection of MRSA

•        In hospitals, patients who have been transferred from another hospital or institution may have swabs taken on admission to screen for MRSA colonisation or infection. The swabs are taken from the nostrils, armpits, groins, genital region and any areas of broken skin (e.g. surgical wounds, ulcers, sores).

•        New or transferring hospital staffs are also screened.

Treatment of MRSA

Vancomycin, Teicoplanin, Clindamycin, Fluoroquinolones, Minocycline.

Control of MRSA

•        Isolation of the patient to prevent spread of MRSA.

•        Strict aseptic techniques.

•        Meticulous hand washing before and after any contact with an infected person, and after handling contaminated equipment or removing gloves.

•        The use of barriers (gloves, gowns, masks) in the appropriate situations when soiling is likely.

•        In some situations, bathing patient with chlorhexidine or use of mupirocin may be advised.