Identification of oxacillin-resistant staphylococci in dogs with end-stage otitis

Autor: Daniel D. Smeak, A. Hillier, Joseph J. Kowalski, Kenneth W. Kwochka, Lynette K. Cole
Rok vydání: 2006
Předmět:
Zdroj: The Veterinary record. 159(13)
ISSN: 0042-4900
Popis: STAPHYLOCOCCAL organisms resistant to both oxacillin and methicillin have historically been referred to as methicillin-resistant staphylococci. However, these organisms are now more correctly referred to as oxacillin-resistant staphylococci, as oxacillin is more stable and appears to be superior for detecting resistance than the other penicillinase-resistant penicillins, methicillin and nafcillin (National Committee for Clinical Laboratory Standards [NCCLS] 1997, Swenson and others 2003, Diekema and others 2004). Oxacillin resistance is initially screened for by using the standard disc diffusion test (DDT) with oxacillin discs, then confirmed by the oxacillin salt agar (OSA) screening test (NCCLS 1997, Swenson and others 2003). Most studies documenting the presence of oxacillin-resistant staphylococcal organisms in the ear do not describe the use of the OSA screening test (Guedeja-Marron and others 1997, 1998, Kiss and others 1997, Cole and others 1998, Lilenbaum and others 2000, Peterson and others 2002), and thus true oxacillin resistance was not verified, except in the study by Gortel and others (1999). In that study, 36 of 51 (71 per cent) staphylococcal organisms were found to be oxacillin resistant by the DDT; however, only 25 of 51 (49 per cent) were oxacillin resistant by the OSA screening test (Gortel and others 1999). Of the 25 isolates of oxacillin-resistant staphylococci identified, seven (28 per cent) were obtained from ear samples (Gortel and others 1999). The identification of true oxacillin resistance is of clinical importance in the selection of antimicrobial therapy for treatment of otitis externa. Oxacillin-resistant staphylococcal organisms are considered resistant to the beta-lactam antimicrobial agents, regardless of their in vitro susceptibility (Brown 2001). It is important to use the OSA screening test to eliminate false-positive oxacillin resistance (that is, apparent resistance to oxacillin and thus all beta-lactam antimicrobials) that might result in incorrect interpretation of antimicrobial susceptibility test results. This short communication describes a study to identify apparent (by the DDT) and true (by the OSA screening test) oxacillin-resistant staphylococcal organisms in the ear canal of dogs with chronic end-stage otitis externa. Twenty-six dogs with chronic end-stage otitis externa undergoing an elective unilateral total ear canal ablation at the Ohio State University Veterinary Teaching Hospital were studied. All the dogs had received topical and systemic medication before entering the study, but none had any topical or systemic antimicrobial agent administered on the day of entry into the study. Before each dog was anaesthetised for surgery, a swab sample was obtained, using a sterile culturette, from one external ear canal from each dog, for bacterial culture and susceptibility testing. The sample was routinely plated on sheep blood agar and MacConkey’s agar and incubated at 35°C for 18 to 24 hours. Staphylococcal organisms were identified on the basis of pigmentation, haemolytic pattern, catalase reaction, coagulase test, Gram stain reaction, and cellular morphology (Bannerman 2003). Antimicrobial susceptibilities were assessed using the standardised DDT and the results were judged as susceptible, intermediate or resistant (Acar and Goldstein 1996). Thirteen antimicrobial agents were tested: ampicillin, amoxicillin-clavulanic acid, cephalothin, chloramphenicol, ciprofloxacin, enrofloxacin, gentamicin, neomycin, oxacillin, polymyxin B sulphate, ticarcillin, tobramycin and trimethoprim-sulfadiazine. If the staphylococcal organism was resistant to oxacillin on the standardised DDT, indicating apparent oxacillin resistance, resistance was confirmed with the OSA screening test (Isenberg 1998). If true oxacillin resistance was recorded on the basis of the OSA test, the organism was reported as being resistant to ampicillin, amoxicillin-clavulanic acid and cephalothin, regardless of the in vitro DDT results with these antimicrobial agents. Twenty-three staphylococcal organisms, 21 Staphylococcus intermedius and two coagulase-negative staphylococci (CNS), were isolated from the external ear canal of 17 of the 26 (65·4 per cent) dogs. Based on the results of the oxacillin DDT, 15 staphylococcal isolates (13 S intermedius and both CNS) were susceptible, one S intermedius was of intermediate susceptibility and seven S intermedius were resistant to oxacillin. However, when oxacillin resistance was verified with the OSA screening test, the S intermedius isolate that was of intermediate susceptibility and five of the seven isolates that were resistant on the oxacillin DDT, were susceptible to oxacillin on the OSA. The susceptibility of the oxacillin-susceptible staphylococcal organisms to the other 12 antimicrobial agents was as follows: 10 were susceptible to all 12 antimicrobial agents, one was susceptible to 11 antimicrobials, three were susceptible to 10 antimicrobials, six were susceptible to eight antimicrobials, and one was susceptible to seven antimicrobials. The susceptibility of the oxacillin-resistant staphylococcal organisms to the other 12 antimicrobial agents was as follows: one was resistant to six antimicrobials and the other was resistant to nine antimicrobials. On the basis of the results of this study, and in accordance with the results of Gortel and others (1999), it is important to perform the OSA screening test to confirm apparent oxacillin resistance of staphylococcal organisms. In the present study, the oxacillin DDT identified seven (30·4 per cent) of 23 staphylococcal organisms to be apparently resistant to oxacillin. However, when the OSA screening test was used to confirm oxacillin resistance, only two (8·7 per cent) of the staphylococcal organisms were truly resistant. Five of seven staphylococcal organisms would have erroneously been reported as resistant to oxacillin as well as resistant to the beta-lactam antibiotics, which may have affected the choice of antibiotic and the outcome of treatment of the otitis.
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