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Decomposition of Casein by Caseobacter polymorphus Strain 253

Laboratory of Microbiology, Agricultural University, Wageningen, The Netherlands

ABSTRACT

Caseobacter polymorphus strain 253 belongs to the nonorange coryneforms predominant on Limburger and similar soft cheeses and is thought to be involved in ripening processes by way of casein decomposition. During growth on sodium caseinate in an otherwise mineral salt medium, endopeptidase, aminopeptidase, and iminopeptidase were produced by the organism. Endopeptidase was extracellular and optimally active at pH 6.5 to 7.0 and 60°C. Its formation roughly parallels growth of the bacterium on sodium caseinate and is inhibited by amino acids, in particular glutamic acid, and by utilizable carbon sources, particularly lactose and glycerol. Aminopeptidase was optimally active at pH 7.35 and 40°C and iminopeptidase at pH 7.45 and 50°C. Their activities increased only slightly during the log phase but sharply at the onset of the stationary phase. Their extracellular log-phase activities were elevated sharply by glutamic acid but iminopeptidase only by a mixture of amino acids. These three proteases are regulated independently.

Ultrasonic treatment of early log-phase cells stimulated their aminopeptidase activity. The L-proline-4-nitroanilide-cleaving activity of intact cells was similar to the activity of sonicated cells. Particularly in early log-phase cultures there were only minor extracellular aminopeptidase and iminopeptidase activities (5 to 15% of the cell-bound activities). At least part of the caseinate is entering the cells as peptides instead of single amino acids.