Bacterial resistance to biocides has not been investigated as extensively as resistance to antimicrobial agents. However, the use of biocides in the prevention of infection is increasingly important. In addition, cross resistance to antimicrobial agents and biocides may develop in some strains. The aim of this study was to determine the MICs of three commonly-used biocides for three groups of staphylococci isolated from cats and dogs: MRSA, MSSA and S intermedius. The null hypothesis was that there would be no difference in the MICs of the 3 biocides between the groups of isolates tested.

Two hundred and four isolates of Staphylococcus aureus (105 methicillin-sensitive Staphylococcus aureus (MSSA) and 99 methicillin-resistant Staphylococcus aureus (MRSA)) and 83 isolates of Staphylococcus intermedius (SI) were used along with two reference strains of S. aureus, ATCC 25923 and NCTC 6571. MICs of povidone iodine, chlorhexidine and triclosan were determined by the agar incorporation doubling dilution method according to NCCLS guidelines.

The MIC50 and MIC90 for povidone iodine were 1:30 for all S aureus and S intermedius isolates. Only 17 isolates were not inhibited by dilutions up to 1:30, and these isolates comprised 1 MRSA and 16 MSSA isolates. The MICs for the reference strains was 1:30. The MIC50 and MIC90 for chlorhexidine were both 1 mg/l for MSSA and 2 mg/l for MRSA and were 0.5 mg/l and 1 mg/l respectively for S intermedius. Three MRSA isolates had an MIC of 4 mg/l. The MICs for the reference strains was 1 mg/l (ATCC 29213) and 0.5 mg/l (NCTC 6571). The MIC50 for triclosan for both MSSA and MRSA was 0.015 mg/l, and for S intermedius was 0.3 mg/l. The MIC90 was 0.5 mg/l for MSSA, but 0.015 mg/l for MRSA (32 fold less than MSSA), and was 0.06 mg/l for S intermedius. Fifteen MSSA isolates had an MIC of 0.25 mg/l or greater, whereas only 2 MRSA isolates did, and no S intermedius isolates did. The MICs for the reference strains were 0.015 mg/l.

A reduced level of susceptibility was seen in a small number of MRSA isolates with chlorhexidine and in a small number of MSSA isolates to triclosan. These concentrations are less than the concentrations used clinically, but may reflect the level of exposure of the micro-organisms to the biocides in the hospital environment. The relationship between antimicrobial resistance and biocide insensitivity will be studied.