Molecular epidemiology of mastitis pathogens of dairy cattle and comparative relevance to humans

Abstract

Mastitis, inflammation of the mammary gland, can be caused by a wide range of organisms, including gram-negative and gram-positive bacteria, mycoplasmas and algae. Many microbial species that are common causes of bovine mastitis, such as Escherichia coli, Klebsiella pneumoniae, Streptococcus agalactiae and Staphylococcus aureus also occur as commensals or pathogens of humans whereas other causative species, such as Streptococcus uberis, Streptococcus dysgalactiae subsp. dysgalactiae or Staphylococcus chromogenes, are almost exclusively found in animals. A wide range of molecular typing methods have been used in the past two decades to investigate the epidemiology of bovine mastitis at the subspecies level. These include comparative typing methods that are based on electrophoretic banding patterns, library typing methods that are based on the sequence of selected genes, virulence gene arrays and whole genome sequencing projects. The strain distribution of mastitis pathogens has been investigated within individual animals and across animals, herds, countries and host species, with consideration of the mammary gland, other animal or human body sites, and environmental sources. Molecular epidemiological studies have contributed considerably to our understanding of sources, transmission routes, and prognosis for many bovine mastitis pathogens and to our understanding of mechanisms of host-adaptation and disease causation. In this review, we summarize knowledge gleaned from two decades of molecular epidemiological studies of mastitis pathogens in dairy cattle and discuss aspects of comparative relevance to human medicine.

Figure 1

Random amplified polymorphic DNA profiles of Serratia marcescens isolates from two dairy herds (indicated by lines). Lanes 1 and 16 contain molecular markers (MM); Lanes 2 and 10 represent teat dip isolates (TD); Lanes 3 through 9 and 11 through 15 represent isolates from composite cow milk samples. Within-herd homogeneity and between-herd heterogeneity are seen

Figure 2

Frequency distribution of sequence types (ST) of Streptococcus agalactiae isolates from bulk tank milk originating from 111 dairy farms in Denmark, 2009, showing predominance of ST1, ST23 and ST103

Figure 3

Pulsed field gel electrophoresis results for staphylococci from teat skin (TS), prepartum secretion (PS) and early lactation milk (M) of a single animal. Lanes 1 and 12 contain molecular markers (MM); Lane 2 and 3 represent Staphylococcus devriesei (Sd) from the left rear (LR) mammary quarter; Lanes 4 to 8 represent Staph. chromogenes (Sc) from the right rear (RR) and right front (RF) quarters; Lanes 9 to 11 represent Staph. aureus (Sa) from the right front quarter. One strain of Staph. devriesei, four strains of Staph. chromognes and two strains of Staph. aureus can be seen (Ringen and Middleton, unpublished data)