Effect of lysin-producing phages in dairy fermentations

Because phage lysin has a much broader lytic range than phage, infection of paired and multi-strain cultures with a lysin-producing phage has the potential to cause fermentation failure and consequent economic loss.

The effect of infecting paired-strain cultures with ØC2 (W) is shown in table 2. Acid production was markedly inhibited for six of the eight combinations. Extensive replication of ØC2 (W) occurred in all phage-infected cultures. Phage infection did not inhibit acid production when the component strains were inoculated singly (in the absence of ØC2) and infected in control experiments. Cell numbers in the inhibited cultures were low at 6 h incubation and ranged from 0.57-30 x 10⁶ colony forming units (CFU/mI). In control cultures, counts ranged from 1-3 x 10⁹ CFU/ml.

Table 2. Effect of infecting paired phage-unrelated single-strain starters with ØC2 (W)*

pH at 6 h
	Phage	Phage	PFU./ml at
Culture	absent	present	6 h (x 10⁸)
C2+Cl0	4.83	6.16	98
C2+ML8	4.85	6.22	117
C2+SCl	5.0	6.15	133
C2+C6	4.91	6.18	154
C2+AM2	4.79	4.98	276
C2+SK11	4.88	5.63	81
C2 + E8	5.13	5.78	79
C2+ US3	4.93	5.25	61

* Lysin was inactivated by heating phage preparations to 57 °C for 1 min., which did not affect the phage titre. Sufficient phage was added to give 1.3 x 10⁴ plaque forming units PFU/ml.)/ml.

The effect of infecting multistrain cultures with C2 (W) is shown in table 3. It is apparent that acid production was inhibited in cultures containg 5% v/v host.

Table 3. Relationship between the C2 cell content of a multi-strain culture and acid production as revealed by the pH attained after 5 h at 30°C in cultures infected and not infected with C2( W)

	Phage absent
% C2 in culture:	1	5	10	20	50
pH at 5 h:	5.61	5.64	5.62	5.35	5.60
	Phage present
% C2 in culture:	1	5	10	20	50
pH at 5h:	5.60	5.86	5.94	5.97	6.11

Reconstituted skim milk was inoculated with multi-strain cultures containing Streptococcus lactis strains C2, ML8 and C10. These contained different levels of C2 as shown above. The initial phage level in infected cultures was 1.4 x 10⁴ pfu/ml. Initial pH of inoculated milk was 6.5, Free lysin was inactivated in phage preparations as indicated in Table 1. From Mullan and Crawford (1985a)

These results show a limitation to the use of paired or multi-strain cultures in phage control schemes: if one of the component strains is attacked by a lysin-producing phage and the other component strains are lysin-sensitive, inhibition of acid production may occur. Since lysin-producing phages constitute a significant part of the phage flora of cheese plants their existence should be considered in selection procedures used to formulate paired and multi-strain starters. Current methods of selection do not include lysin sensitivity tests. The formulation of lysin-insensitive strains which do not host lysin-producing phages from strains which also comply with existing strain selection criteria should give cultures of increased insensitivity to phage. Because lysin and phage levels are related, controlling phage levels in cheesemaking plants, using proven methods of phage control, should minimize the inhibitory effects of lysins.

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Assay of phage lysins

Isolation and purification of phage lysins

Characterisation of phage lysins

Role of phage lysin in intracellular phage release

EFFECT OF LYSIN-PRODUCING PHAGES IN DAIRY FERMENTATIONS