In vitro antimicrobial activity of polyhexamethylene guanidine hydrochloride and polyhexamethylene biguanide against microorganisms causing bovine mastitis.

Bovine mastitis not only affects the health of cows but also causes economic losses and public health concerns due to the presence of zoonotic and resistant microorganisms in milk and its derivatives. Thus, the present study aimed to evaluate the in vitro antimicrobial activity of polyhexamethylene guanidine hydrochloride (PHMGH) and polyhexamethylene biguanide hydrochloride (PHMB) against standard and clinical isolated bacteria strains that causes clinical mastitis (Staphylococcus aureus ATCC 6538 and clinical isolate, Staphylococcus epidermidis ATCC 12228, Streptococcus agalactiae ATCC 12386, Streptococcus dysgalactiae ATCC 12238, and Escherichia coli ATCC 14948 and clinical isolate). The polymers were tested at concentrations ranging from 0.3 ppm to 5000 ppm and compared to the commercial product benzalkonium chloride (BKC). The minimum inhibitory concentration (MIC) technique determined the lowest concentration of polymers capable of inhibiting microbial growth, while the minimum bactericidal concentration (MBC) characterized the lowest concentration with bactericidal or bacteriostatic action. Fractional inhibitory concentration index (FICI) assessed the synergy of polymers with BKC. To determine the time required for microbial death in the presence of polymers and BKC, the bactericidal kinetics technique was employed, and the minimum inhibitory concentration for biofilm inhibition (CIMB75) assessed the lowest concentration of polymers inhibiting 75% or more of biofilm formation. The data indicated that both PHMGH and PHMB were effective against all tested microorganisms at concentrations ranging from 0.3 to 9.6 ppm, as determined by MIC and MBC. FICI showed indifferent effects of polymer combinations with BKC against most microorganisms, except for PHMGH against Staphylococcus aureus, which exhibited additive effects. Bactericidal kinetics demonstrated that polymers immediately reduced microbial load by more than 90% against tested microorganisms, unlike BKC. CIMB75 showed that polymers inhibited 75% of biofilm formation, with PHMGH proving more effective. Based on these results, synthetic polymers PHMGH and PHMB showed promising in vitro antimicrobial effects against pathogens causing bovine mastitis, suggesting potential for developing new preventive products for this mammary condition.