title: Effect of antibiotics on the bacterial load of meticillin-resistant Staphylococcus aureus colonisation in anterior nares.
abstract: Prevalence of hospital-acquired meticillin-resistant Staphylococcus 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 loads 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 2-5log(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.
authors.affiliation: Department of Microbiology, Research Centre of Infection and Immunology, The University of Hong Kong, Hong Kong SAR, China.
authors.forename: V C C
authors.initials: VC
authors.lastname: Cheng
authors.forename: I W S
authors.initials: IW
authors.lastname: Li
authors.forename: A K L
authors.initials: AK
authors.lastname: Wu
authors.forename: B S F
authors.initials: BS
authors.lastname: Tang
authors.forename: K H L
authors.initials: KH
authors.lastname: Ng
authors.forename: K K W
authors.initials: KK
authors.lastname: To
authors.forename: H
authors.initials: H
authors.lastname: Tse
authors.forename: T L
authors.initials: TL
authors.lastname: Que
authors.forename: P L
authors.initials: PL
authors.lastname: Ho
authors.forename: K Y
authors.initials: KY
authors.lastname: Yuen
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D019072:Antibiotic Prophylaxis; D002353:Carrier State / Q000382:microbiology*; D016000:Cluster Analysis; D015169:Colony Count, Microbial; D003428:Cross Infection / Q000382:microbiology; D016172:DNA Fingerprinting; D004269:DNA, Bacterial / Q000235:genetics; D016521:Electrophoresis, Gel, Pulsed-Field; D004783:Environmental Microbiology; D005260:Female; D006801:Humans; D008297:Male; D016106:Methicillin Resistance; D008875:Middle Aged; D009666:Nose / Q000382:microbiology*; D011446:Prospective Studies; D013203:Staphylococcal Infections / Q000382:microbiology; D013211:Staphylococcus aureus / Q000145:classification / Q000187:drug effects* / Q000302:isolation & purification*; D013997:Time Factors
chemical_list: D000900:Anti-Bacterial Agents; D004269:DNA, Bacterial