Growth and Adherence on Stainless Steel by Enterococcus faecium Cells†

Abstract

Enterococcus faecium isolated from Brazilian raw milk was used in this study. For growth studies, E. faecium was inoculated into 10% RSM (reconstituted skim milk) and MRS broth, incubated at 6.5 and 9°C for 10 days and at 30, 42, and 45°C for 48 h. Cells were enumerated after spread-plating onto MRS agar and incubating at 30°C for 48 h. The ability of E. faecium cells to adhere to stainless-steel chips (6 by 6 by 1 mm, AISI 304, finish #4) was investigated. MRS broth containing stainless Steel chips was inoculated to an initial concentration of 103 or 106 CFU/ml of E. faecium. Adherent cells were stained with acridine orange and enumerated by epifluorescence microscopy. E. faecium grew between 6.5 and 42°C in MRS and between 9 and 40°C in RSM. In MRS broth with 106 or 103 CFU/ml, the g (generation time) values were 0.62 and 0.42 h and <I>R</I> (growth rate) values were 1.6 and 2.4 h−1. Values of <I>R =</I> 2.3 h−1 and g = 0.43 h were determined for E. faecium growing in RSM with 103 CFU/ml. In MRS broth, for samples with a starting concentration of 106 cells per mi, adherence to stainless-steel chips was first observed at 2 h. However, adherence was first observed at 4 h in samples with an initial concentration of 103 cells per mi. After 10 h of exposure the number of adherent cells was similar for all samples regardless of initial inoculum. These results indicate that E. faecium readily adheres to stainless Steel. It also underscores the need to control E. faecium by using appropriate low storage temperatures and adequate sanitizing practices in the dairy industry.