Association between phage types and antimicrobial resistance among bovine Staphylococcus aureus from 10 countries

Abstract

This study was conducted to investigate the diversity of phage types and associations between penicillin resistance and phage types among 815 Staphylococcus aureus isolates from bovine mastitis in nine European countries and USA. All isolates were examined for susceptibility to antimicrobial agents and characterised by phage typing. Penicillin resistance was found among strains from all countries with an average occurrence of 32.4% (2–71.4%). A total of 76% of isolates were identifiable by phage typing and 144 different phage types were observed. The most predominant types were phage type 29 (11% of the 815 isolates), phage type 52 (5%), and phage type 80 (5%). Phage type 95 and 29/52/52A/80 were both distributed within seven countries. In the countries with the highest occurrence of penicillin resistance a reduced diversity of phage types and phage groups was observed. Phage group III was significantly associated with penicillin resistance in contrast to phage group I (P=0.0023) and phage complex-80 (P=0.0066). This study confirms that a large number of phage types of S. aureus cause bovine mastitis, but that some types predominate. In addition, these findings could indicate that the use of penicillin in the bovine environment has selected for specific types of S. aureus in countries with a high frequency of resistance.

Introduction

Bovine mastitis is the most common production disease in dairy herds worldwide. It causes significant economic losses due to rejected milk, reduced milk quality, early culling, drug costs, veterinary expenses and increased labour costs for the farmer (Hoblet et al., 1991, Gruet et al., 2001), and it is the most frequent reason for antimicrobial use on dairy farms (Erskine, 2000). The most common cause of bovine mastitis in Denmark and other countries is Staphylococcus aureus (Myllys et al., 1994, Waage et al., 1999). The outcome for treatment of mastitis caused by S. aureus is variable and cure rates are not good, primarily due to the poor distribution of the drug in the inflamed udder and the occurrence of antimicrobial resistant staphylococci (Pyorala and Pyorala, 1994, Gruet et al., 2001). Penicillins have been the drugs of choice for treatment of bovine mastitis since its introduction in the late 1940s. However, soon after its introduction, penicillin resistant bovine S. aureus were recorded and in certain geographical areas the level of penicillin resistance was up to 70% through the 1960s (Wilson, 1961). An increase in occurrence of penicillin resistance has been observed over time in most countries, but major differences between countries can be observed (Aarestrup and Jensen, 1998). These geographical differences and fluctuations may reflect the occurrence of different clones of S. aureus within each country and might be related to the national policy for use of antimicrobial agents.

A large number of different phenotyping and genotyping techniques for characterisation of S. aureus are available (Schlichting et al., 1993, Fitzgerald et al., 1997, Fitzgerald et al., 2001, Lange et al., 1999). In previous studies good accordance between different genotyping techniques and phage typing in characterisation of bovine S. aureus has been found (Schlichting et al., 1993, Aarestrup et al., 1995b), and for large scale screening phage typing has previously proved valuable for both human and bovine strains (Larsen et al., 2000, Zadoks et al., 2000).

Several studies have characterised bovine S. aureus within countries and identified clustering or clonal relationships between isolates (Lange et al., 1999, Raimundo et al., 1999, Su et al., 2000, Tollersrud et al., 2000, Zadoks et al., 2000, Buzzola et al., 2001) but only a limited number of studies have compared bovine strains from different countries (Kapur et al., 1995, Fitzgerald et al., 1997, De Oliveira et al., 1999).

This study was conducted to investigate the diversity of phage types and associations between penicillin resistance and phage types among 815 S. aureus isolates from bovine mastitis in 10 countries.