# In vitro efficacy of four potential antimicrobial substances against Klebsiella species and Streptococcus uberis isolated from bovine mastitis

## TL;DR

This study tests four potential antimicrobial substances against bacteria causing bovine mastitis, finding them effective even against antibiotic-resistant strains.

## Contribution

The study evaluates the in vitro efficacy of four novel antimicrobial substances against antibiotic-resistant mastitis-causing bacteria.

## Key findings

- MGO showed the lowest MICs against both Klebsiella spp. and Streptococcus uberis.
- All four substances inhibited bacterial growth, even in antibiotic-resistant isolates.
- PVP iodine, NAC, NCT, and MGO could be considered as potential alternatives for mastitis treatment.

## Abstract

Mastitis is an important disease in dairy cattle with huge influence on animal welfare and economic situation of farmers. With very limited choices in antibiotic therapy of bovine mastitis and the relevance of emerging antimicrobial resistances, discovery of new antimicrobials is one of the major challenges for veterinary pharmacological research. Thus, in vitro efficacy of four promising substances was investigated: polyvinylpyrrolidone iodine (PVP iodine), N-acetylcysteine (NAC), N-chlorotaurine (NCT), and methylglyoxal (MGO). PVP iodine is an antiseptic commonly used in all medical fields. The two amino acid derivates NAC and NCT are a potent antioxidant respective oxidant with antimicrobial properties in various indications. MGO is one of the active components in Manuka honey, causing bacterial death by repression of protein and nuclei acid synthesis. For experiments, Klebsiella species (spp.) and Streptococcus (S.) uberis were isolated from bovine milk samples. Subsequently, broth microdilutions with common antibiotic agents and the four potential antimicrobials for determination of minimal inhibitory concentrations (MICs) and examinations regarding minimal bactericidal concentrations (MBCs), also time-dependent, were performed according to CLSI standards. Regarding antimicrobial susceptibility, growth of Klebsiella spp. isolates was not restrained by examined concentrations of benzylpenicillins, aminopenicillins, isoxazolyl penicillins, macrolides, and lincosamides. However, studied concentrations of aminopenicillins combined with beta-lactamase inhibitors, cephalosporins (partly combined with aminoglycosides) as well as fluoroquinolones inhibited their growth. In contrast, all S. uberis isolates were susceptible to tested concentrations of penicillins, cephalosporins, macrolides, lincosamides, and fluoroquinolones. Broth microdilutions of the four substances revealed median MICs of PVP iodine with 5 mg/mL for Klebsiella spp. and 4 mg/mL for S. uberis and NAC with MICs of 5 mg/mL for Klebsiella spp. and 1.75 mg/mL for S. uberis. MICs of NCT were 1.5 mg/mL for Klebsiella spp. and 1 mg/mL for S. uberis. The lowest MICs were found for MGO with 0.4 mg/mL for Klebsiella spp. and 0.1375 mg/mL for S. uberis. Growth of every bacterial isolate was inhibited through all four substances, even when resistant to commonly used antibiotic agents. Therefore, with positive findings in further experiments regarding biocompatibility and efficacy, they could be considered as auspicious alternatives for mastitis treatment in dairy cattle.