Growth characteristics of Staphylococcus aureus and Escherichia coli in whey from sequentially infected milk.

The growth of Staphylococcus aureus and Escherichia coli was followed in bovine whey samples which had been prepared from milk previously incubated with cultures of S. aureus or E. coli. Staphylococcal strains were divided into 2 groups according to their ability to form compact or diffuse colonies on serum soft agar, which is related to the absence or presence of capsule respectively. The growth of compact staphylococci was dependent on the bulk tank milk used whereas diffuse colony forming staphylococci grew equally well in all bulk milk, also in all inoculated milk. The growth of E. coli was markedly enhanced in whey samples prepared from milk preincubated with staphylococci. However, clear growth inhibition was seen with E. coli and S. aureus strains when grown in whey prepared from milk preincubated with E. coli. Results indicate that the growth promotion of pathogens due to compositional changes in milk are of importance during the course of infection because the growth pattern on staphylococci is dependent on these compositional changes. The growth-inhibitory effects caused by E. coli may explain difficulties in isolating this organism.