Effect of antibiotics on the bacterial load of meticillin-resistant Staphylococcus aureus colonisation in anterior nares

Prevalence of hospital-acquired meticillin-resistant Stophylococcus aureus (MRSA) infection or colonisation has been associated with antimicrobial consumption. The impact of antibiotic treatment on nasal. colonisation is unknown. We conducted a three-month prospective study of 116 patients with extranasal MRSA infection or colonisation, whose nasal. MRSA bacterial, loads were determined during and after various antibiotic courses over a period of three weeks. Environmental swabs were also taken from the near patient environment. Concomitant nasal MRSA carriage was observed in 76.7% of extranasal MRSA-colonised or -infected patients. The median nasal MRSA bacterial load increased significantly from 2.78 (range 0-6.15) to 5.30 (range 2.90-8.41) log(10) cfu per swab (cfu/swab) (P < 0.001) over 21 days during beta-lactam therapy. It also increased from 0 (range 0-4.00) to 4.30 (range 0-7.46) log(10) cfu/swab (P=0.039) over 14 days during fluoroquinolone therapy. Median bacterial. toads were significantly higher for beta-lactam- and fluoroquinolone-treated patients on day 7 [4.78, range 0-7.30], day 14 [4.30, range 0-7.60] and day 21 [5.30, range 2.90-8.41] than controls not receiving antibiotics (P < 0.05). These loads then decreased by 2-5 log(10) cfu/swab 2 weeks after discontinuation of antibiotics. The environment of patients receiving beta-lactam agents (relative risk: 3.55; 95% confidence interval: 1.30-9.62; P = 0.018) or fluoroquinolones (4.32; 1.52-12.31; P = 0.008) demonstrated more MRSA contamination than the environment around control patients (0.79; 0.67-0.93; P = 0.002). Patients on beta-lactam or fluoroquinolone therapy have increased incidence of MRSA colonisation and higher nasal bacterial. Loads, and appear to spread their MRSA into the near patient environment. (C) 2008 The Hospital Infection Society. Published by Elsevier Ltd. All rights reserved.