Changes occurring in freshwater ecosystems seem to be fundamental in the development of all microorganisms, including those pathogenic to fish. This has been especially evident in recent years during which dynamic variations in bacterial fish pathology have been observed. Gram-negative bacteria commonly known to be pathogenic to fish, like Aeromonas spp., Flavobacterium spp., Pseudomonas spp., and Shewanella putrefaciens are replaced by other species, which until now have not been known to be virulent or even conditionally pathogenic to fish. Nowadays, among these other species Acinetobacter spp., Plesiomonas shigelloides, Sphingomonas paucimobilis, and Stenotrophomonas maltophilia are the most frequently isolated from fish exhibiting clinical signs of disease. Two Gram-positive bacteria have become pathogens of particular importance in fish pathology in Poland: Lactococcus garviae and Streptococcus iniae. In addition, infections caused by the Gram-positive bacterium Kocuria rhizophila have appeared in recent years. This bacterium has not been known until now to be pathogenic to fish. Therefore, this infection could be called an emergent disease.

In vitro testing of bacterial susceptibility to a combination of ticarcillin and clavulanic acid was done, using 406 aerobic gram-positive and gram-negative isolates (considered to be pathogens) cultured from equine and small animal specimens. A microdilution broth technique of susceptibility testing was performed, using trays with wells containing a range of doubling concentrations of dehydrated ticarcillin (range, 0.50 to 128 microgram/ml) with fixed concentration of clavulanic acid (4 microgram/ml). The following isolates of equine origin were (90%) susceptible to concentrations of ticarcillin and clavulanic acid combinations of less than or equal to 16 and 4 microgram/ml, respectively: Staphylococcus aureus, S intermedius, Klebsiella pneumoniae, Enterobacter aerogenes, Ent agglomerans, Ent cloacae, Escherichia coli, Actinobacillus sp, Corynebacterium pseudotuberculosis, Rhodococcus equi, Proteus vulgaris, and Bordetella bronchiseptica. Isolates of small animal origin (90%) susceptible to less than or equal to 16 and 4 microgram of ticarcillin-clavulanic/ml included S aureus, S intermedius, Ent aerogenes, Ent agglomerans, Pasteurella multocida, B bronchiseptica, Pr mirabilis, and Serratia sp.