METHICILIN RESISTANT STAPHYLOCOCCUS AUREUS IN CLINICAL SAMPLES AND FOMITES

CHAPTER ONE

1.0    INTRODUCTION

1.1 BACKGROUD OF STUDY

Methicillin-resistant Staphylococcus aureus (MRSA) are strains of Staphylococcus aureus which are resistant to methicillin and related penicillins and are particularly difficult to treat because they are also resistant to most other common antibiotics (Cheesbrough, 2000).

Although Staphylococcus aureus infections were historically treatable with common antibiotics, emergence of drug-resistant organisms is now a major concern. MRSA was endemic in hospitals by the late 1960s, but it appeared rapidly and unexpectedly in communities in the 1990s and is now prevalent worldwide (Deleo, 2009; Liebowitz, 2009). Staphylococci are gram positive cocci of uniform size, occurring characteristically in groups but also singly and in pairs. They are non-motile and non-capsulated (Cheesbrough, 2000). Staphylococcus aureus is the most medically important member in terms of pathogenicity of the group (Ochei and Kolhatkar, 2000).

Staphylococcus is present in the nose of 30% of healthy people and may be found on the skin. It causes infection most commonly at sites of lowered host resistance, such as damaged skin or mucous membrane (Humphrey, 2007). Although 50 – 60% of patients with MRSA are merely colonised (i.e. they carry the bacteria but do not have symptoms or an illness), serious infections such as those involving the blood stream, respiratory tract and bones or joints do occur (Humphrey, 2007). S. aureus causes boils, pustules, styes, impetigo, infections of wounds (cross-infections), ulcers and burns, osteomyelitis, mastitis, septicaemia, meningitis, pneumonia and pleural empyema. Also, toxic food poisoning (rapid onset, no fever), toxic shock symdrom and toxic skin exfoliation (Chessbrough, 2000).

Mannitol salt agar is a useful selective medium for recovering S. aureus from faecal specimens when investigating staphylococcal food poisoning. It can also be used to screen for nasal carriers. S. aureus ferments mannitol and is able to grow on agar containing 70 – 100g/l sodium chloride. Mannitol salt agar containing 75g/l sodium is recommended particularly for isolating MRSA strains (Cheessbrough, 2000).

On mannitol salt agar, S. aureus produces yellow colonies (Ochei and Kolhatkar, 2000). The MRSAs are usually sensitive to vancomycin (Ochei and Kolhatkar, 2000). Flucloxacillin and chloxacillin are used to treat b-lactamase (penicilinase) producing staphylococci. Vacomycin is often needed to treat MRSA infections. Antibacterial resistance to penicillin may occur due to the b-lactamase production, cell membrane alterations reducing antibiotic uptake (gram negative bacteria), or changes in the penicillin-binding protein as occurs with MRSA (Cheesbrough, 2000).

There is no effective immunisation with toxoids or bacterial vaccines for preventing the spread of S. aureus (Levinson and Jawetz, 2002). The control and prevention of MRSA involves early and reliable detection in the laboratory through surveillance, patient isolation when admitted to hospital, good professional practice by all healthcare workers (including compliance with hand hygiene guidelines), effective hospital hygiene programmes and sensible use of antibiotics (Humphrey, 2007).