Megasphaera

A genus of strictly anaerobic, spheroid, Gram-negative bacteria in the family Veillonellaceae, Megasphaera includes species — notably M. cerevisiae — that spoil beer, generating hydrogen sulphide and butyric off-flavours plus turbidity and ropiness. These anaerobes thrive in the low-oxygen environment of packaged beer, often arriving via contaminated pitching yeast or biofilms. Breweries counter them with hygienic design, effective cleaning-in-place, pasteurisation or sterile filtration of bright beer, and routine microbiological checks of yeast, rinses and finished product so contamination is caught before release.