CHRISTOPH JANS1, ANDREA GERBER1, JOSÉPHINE BUGNARD1, PATRICK MURIGU KAMAU NJAGE2,1, CHRISTOPHE LACROIX1, LEO MEILE1
1ETH Zurich, Institute of Food, Nutrition and Health, Switzerland
2University of Nairobi, Department of Food Technology, Kenya
Streptococcus infantarius subsp. infantarius, a pathogenic species of the Streptococcus bovis / Streptococcus equinus complex (SBSEC), was unexpectedly found to be predominating in spontaneously fermented East-African camel milk (suusac) over other lactic acid bacteria including Streptococcus thermophilus. Lactose metabolism of currently known SBSEC and the classic yoghurt bacterium S. thermophilus is reported to differ, not justifying the predominance observed. Bacteriocin production by SBSEC is described and might be a factor contributing to the predominance.
African S. subsp. infantarius predominate in suusac fermentation mediated by bacteriocin production and an adapted lactose metabolism that parallels lactose utilisation in S. thermophilus instead of previously reported SBSEC metabolic pathways. Presumptive S. subsp. infantarius isolates (213) were obtained at 108CFUml-1 from 22 out of 24 Kenyan and Somali suusac on semi-selective media and subsequently typed by pheno- and genotypic methods. Primers based on S. thermophilus and S. salivarius sequences were designed for 6 genes of the gal"=lac operon. Genes galE, galM, lacS and lacZ were detected by PCR in African S. subsp. infantarius strains. Sequencing of a 6.949-kb fragment of S. subsp. infantarius CJ18 revealed higher homology to galE (94.1%), galM (92.1%), lacS (93.1%) and lacZ (97.4%) of S. thermophilus than to galE (83.3%) and galM (56.6%) of the S. subsp. infantarius type strain. lacS and lacZ were not detected in the available genome scaffolds and PCR assays of all 9 SBSEC type strains. Bacteriocin screening was performed using a wide spectrum of indicator strains in buffered media. Bacteriocins active against Listeria, lactobacilli and streptococci were detected in 33 out of 66 tested S. subsp. infantarius strains.
Our African S. subsp. infantarius strains harbour genes for a lactose metabolism homologous to those of S. thermophilus, which are either absent or strongly differ in SBSEC type strains. The presence of an adapted gal-lac operon and the production of bacteriocin possibly explain their predominance during suusac fermentation. Our investigations are continuing to better define additional metabolic characteristics, the fermentative role of CJ18 and to identify any putative virulence factors that may pose a significant human health risk.
Keywords: Bacteriocin, camel milk fermentation, galactose lactose metabolism, Streptococcus infantarius