In vitro antimicrobial susceptibilities of threePorphyromonasspp and in vivo responses in the oral cavity of cats to selected antimicrobial agents
Authors: NORRIS JM , LOVE DN 
Published: March 2008

ObjectivesTo determine in vitro susceptibility ofPorphyromonas gingivalis, P salivosaandP circumdentariato seven antimicrobial agents by agar dilution and Epsilometer test methods and to assess the effectiveness of these antimicrobial agents in reducing the numbers of eachPorphyromonasspp in the oral cavity of 16 domestic cats.

DesignA two‐part prospective study involving in vitro antimicro‐bial studies usingPorphyromonasspp obtained from naturally occurring feline infections and in vivo antimicrobial response studies using client‐owned cats with naturally occurring periodontal disease.

ProcedureIsolates (n = 25) of three felinePorphyromonasspp from the oral cavity and oral‐associated disease were tested for their in vitro susceptibility to amoxycillin, amoxycillin‐clavulanate, benzylpenicillin, clindamycin, doxycycline, erythromycin and metronidazole, using agar dilution and Epsilometer test methods. Digoxigenin‐labelled whole chromosomal DNA probes directed againstP gingivalis VPB3492,P circumdentariaNCTC 12469TandP salivosaVPB 3313 were used to quantify organisms taken from two sample sites at the gingival margins of these cats prior to, and 5 days after, treatment with one of four commonly used antimicrobial products (amoxycillin‐clavulanate, clindamycin, doxycycline or spiramycin‐metronidazole). The response to treatment was assessed clinically for each cat.

ResultsAll isolates were susceptible in vitro to all seven antimicrobial agents using both methods. The numbers ofP gingivaliswere not reduced at the gingival sample sites by administration of amoxycillin‐clavulanate for 5 days, although this treatment reduced the numbers ofP salivosaandP circumdentaria tobelow detection levels in six of eight and two of three of sample sites, respectively; clinical improvement was not observed in cats treated with amoxycillin‐clavulanate. Treatment with clindamycin, doxycycline or spiramycin‐metronidazole resulted in clinical improvement and a marked reduction of allPorphyromonasisolates at the sample sites.

ConclusionThe Epsilometer test is a simple and accurate method for determining the minimum inhibitory concentration forP gingivalis, P salivosaandP circumdentaria.All strains were susceptible in vitro to all the antimicrobial agents tested, although clinical improvement of gingival disease was not noted with amoxycillin‐clavulanate when given for 5 days at usual doses. This appears to be the first report of the disparity between the in vivo and in vitro susceptibility of oral bacterial strains to amoxycillin‐clavulanate in the veterinary dental literature. This also appears to be the first report in which clinical and microbiological responses to commonly used antimicrobial agents for periodontal disease in cats has been documented and quantified. It was shown that treatment with clindamycin, spiramycin‐metronidazole or doxycycline not only produced a substantial reduction in the number ofPorphyromonasspp (in the majority of cases to below detection levels), but also resulted in substantial clinical improvement. This would indicate that these antimicrobial agents are useful adjunctive therapy to mechanical debridement in domestic cats.