The association of coryneform bacteria with animal disease has received little attention due to the complexity of their taxonomic status and difficult identification. Two biotyping systems and antimicrobial susceptibility were used to characterize coryneform isolates from otitis externa.

A total of 10 strains were isolated from 9 dogs and 1 cat with otitis externa. Cytology confirmed otitis externa and the predominance of irregularly shaped gram-positive rods. API Coryne and RapID CB Plus were used for species identification. Susceptibility testing was performed by disc diffusion in Muller-Hinton with 5% sheep blood. The following antimicrobials were used: β-lactams (penicillin G P, amoxicillin AML, amoxicillin/clavulanate AMC, cephalothin KF, cefixime CFM, cefuroxime CXM, cefoxitin FOX, ceftazidime CAZ, ceftriaxone CRO), aminoglycosides (gentamicin CN, amikacin AK), clindamycin DA, chloramphenicol C, tetracycline TE, trimethoprim/sulfamethoxazole SXT and fluoroquinolones (enrofloxacin ENR, marbofloxacin MAR, ciprofloxacin CIP, norfloxacin NOR, ofloxacin OFX, orbifloxacin OBX, levofloxacin LEV). NCCLS susceptibility criteria were applied.

API Coryne biotyping system identified 8 of the isolates as Corynebacterium jeikeium, 1 as Corynebacterium striatum, with low level of discrimination. One isolate was not identified. RapID CB Plus biotyping system allowed the identification of the 10 coryneform bacteria as Corynebacterium minutissimum (n=2), Turicella otitidis (n=2), Brevibacterium Group B (n=2), Corynebacterium amycolatum (n=1), Corynebacterium striatum (n=1), CDC Coryneform Group G (n=1) and Rhodococcus equi (n=1) (ancient Corynebacterium equi). Coryneform strains were very susceptible to fluoroquinolones (ENR 80%, MAR 100%, CIP 100%, NOR 90%, OFX 90%, LEV 90%), except for OBX (50%), aminoglycosides (CN 80%, AK 90%), chloramphenicol (70%) and tetracycline (100%). Isolates presented diminished susceptibility to penicillin G (90%) but were highly susceptible to AML, AMC, KF and CXM (90%, 100%, 90% and 80%, respectively). However, resistance was found towards third generation cephalosporins (CFM 90%, CAZ 70%, FOX 30% and CRO 20%), sulphonamides (SXT 60%) and macrolides (DA 40%).

RapID CB Plus biotyping system allowed the correct identification of the 10 coryneform isolates as previously demonstrated by others¹. All coryneform isolates described in this work have been already described as opportunistic human pathogens². Thus, this study demonstrates the association of coryneform bacteria with otitis externa in dogs and cats. Furthermore, susceptibility data demonstrated a concerning level of resistance among coryneform isolates, namely towards third generation cephalosporins and clindamycin, commonly used in the treatment of otitis externa. This study suggests that a relevant role should be attributed to the presence of coryneform bacteria in otitis externa, also in companion animals.

References

1.  Hudspeth, M et al 1998 J. Clin. Microbiol., 36, 543-547.

2.  Funke, G. et al 1997 Clin. Microbiol. Rev., 10, 125-159.