Resistance of gram-negative bacteria to gentamicin has become an increasingly common problem among clinical isolates from human beings. Susceptibility of isolates from horses to gentamicin and amikacin was evaluated for the period from July, 1983 to June, 1985. All isolates of Escherichia coli, and species of Enterobacter, Klebsiella, Proteus, and Pseudomonas examined were susceptible to amikacin, except 2 of the 46 Pseudomonas isolates. In contrast, 13 to 50% of isolates were resistant to gentamicin. Escherichia coli, and Klebsiella, Proteus, and Enterobacter species isolates were highly significantly more susceptible to amikacin (P less than 0.01) than to gentamicin. Pseudomonas spp (P = 0.13) were not significantly different in susceptibility to the 2 drugs. There was significant variation among genera in their susceptibility to gentamicin (P = 0.002), primarily because of the frequency of resistance in isolates of Klebsiella spp and Proteus spp, compared with the other 3 organisms (E coli, Enterobacter spp, and Pseudomonas spp). There was no significant difference of susceptibility to amikacin among the genera studied (P = 0.06).

Polysulfated glycosaminoglycan (PSGAG) recently have been reported to potentiate the infectivity of Staphylococcus aureus in horses with experimentally induced septic arthritis. Four groups of 8 horses each had 1 midcarpal joint injected with approximately 33 viable colony-forming units (CFU) of S aureus plus either 1 ml of saline solution (group 1), 250 mg of PSGAG (group 2), 250 mg of PSGAG passed through a 0.6-micrometer filter (group 3), or 250 mg of PSGAG plus 125 mg of amikacin (group 4). Horses that developed clinical signs consistent with sepsis were euthanatized, and samples were collected at necropsy. Horses that survived had samples obtained by use of arthroscopy at days 13 and 14 after injection. Staphylococcus aureus was isolated from 1 group-1 horse, 8 group-2 horses, and 7 of 7 group-3 horses that met protocol, but was not isolated from any group-4 horses. All 16 aforementioned horses had clinical signs, results of synovial fluid analysis, and gross pathologic and synovial membrane histopathologic findings that were consistent with septic arthritis. Polysulfated glycosaminoglycan (250 mg) increased the infectivity of 33 CFU of S aureus (P = 0.001); filtering the PSGAG had no effect. Intra-articular injection of 125 mg of amikacin immediately after inoculating the joint with 33 CFU of S aureus significantly (P = 0.001) decreased potentiation of infection by the PSGAG.