Introduction

Staphylococcus aureus is commonly carried on the skin or in the nose of healthy individuals. It is an important pathogen in human infections causing illness ranging from minor skin infections and abscesses to life - threatening diseases such as pneumonia, meningitis, endocarditis, toxic shock syndrome and septicaemia which may be rapidly fatal [1]. Bacterial resistance to antibiotics has been recognized since the first drugs were introduced for clinical use. Penicillin was first introduced in 1941, when less than 1% of Staphylococcus aureus strains were resistant to its action. By 1947, 38% of hospital strains had acquired resistance and currently over 90% of Staphylococcus aureus isolates are resistant to penicillin. Increasing resistance to antibiotics is a consequence of selective pressure [2]. In orthopaedics, S. aureus has been implicated in surgical site infection, painful infection of joint fluid known as septic or infective arthritis, post-operative infection, implant devices, infection following trauma, chronic osteomyelitis subsequent to an open fracture, meningitis following skull fracture. Нis study was aimed at determining the antibiotic susceptibility pattern of the S. aureus isolates from orthopaedic patients in a tertiary hospital in North-western Nigeria.

Methodology

Sample collection

One hundred clinical samples were collected aseptically from the wound, skin and bed of orthopaedic patients in Ahmadu Bello University Teaching Hospital Zaria, Nigeria over a period of 5 months. Ethical approval and patients’ consent were obtained.

Identification of S. aureus isolates

API STAPH identification kit (bioMerieux, Inc, Durham, USA) was used to identify the S. aureus isolates. Нe procedures were carried out according to the manufacturer’s instructions. A

ntibiotic susceptibility test

Disk diffusion tests was performed for each of the isolates previously identified as S. aureu follow the method recommended by the Clinical Laboratory Standard Institute [3]. List of antibiotics used are: Cefoxitin 30 µg, &eÑ–ria[one 30 µg, Vancomycin 30 µg, Ampicillin 10 µg, Gentamicin 10 µg, 3eflo[acin 5 µg, &iproflo[acin 5 µg, Amoxicillin-clavulanic acid 30 µg, Erythromycin 15 µg and Clindamycin 2 µg (Oxoid Ltd. Basingstoke, London).

Test for β-lactamase production (nitrocefin test)

Enzyme extracts of the S. aureus isolates were prepared as described by Caddick [4] with modification. Microplate 1itrocefin assay was carried out as follows: 1 mg lyophilized 1itrocefin powder (Oxoid, UK) was reconstituted in 1.9 ml of 0.1 M phosphate buffer pH 7 supplied by the manufacturer. Нe reconstituted nitrocefin was further diluted 1 in 10 with PBS to give 50 µg/ml solution. Нe disrupted cell preparations were used immediately by dispensing 50 µL of preparation into separate wells of a 96 well plate. 50 µL of diluted nitrocefin solution was added into each of the wells and incubated at 37°C for 10 minutes. In the presence of β-lactamase, the chromogenic nitrocefin substrate changes colour from yellow to pink/red. Determination of multiple antibiotics resistance (MAR) index Нe Multiple Antibiotic Resistance (MAR) index was determined for each isolate by dividing the number of antibiotics to which the organisms is resistant to by the total number of antibiotics tested [5-7].