Variables and parameters.

In the dairy industry bacteriophage (phage) contamination significantly impairs the production and quality of products like yogurt and cheese. To combat this issue, the strains of bacteria used as starter cultures possess mechanisms that make them resistant to phage infection, such as envelope resistance, or processes that render them immune to phage infection, such as restriction-modification and CRISPR-Cas. Lactococcus lactis, used to manufacture cheese and other dairy products, can also block the reproduction of infecting phages by abortive infection (Abi), a process in which phage-infected cells die before the phage replicate. We employ mathematical-computer simulation models and experiments with two Lactococcus lactis strains and two lytic phages to investigate the conditions under which Abi can limit the proliferation of phages in L. lactis populations and prevent the extinction of their populations by these viruses. According to our model, if Abi is almost perfect and there are no other populations of bacteria capable of supporting the replication of the L. lactis phages, Abi can protect bacterial populations from succumbing to infections with these viruses. This prediction is supported by the results of our experiment, which indicate that Abi can help protect L. lactis populations from extinction by lytic phage infections. However, our results also predict abortive infection is only one element of L. lactis defenses against phage infection. Mutant phages that can circumvent the Abi systems of these bacteria emerge. The survival of L. lactis populations then depends on the evolution of envelope mutants that are resistant to the evolved host-range phage.

在乳制品工业领域，噬菌体（bacteriophage，简称phage）污染会显著降低酸奶、奶酪等产品的生产效率与品质。为应对该问题，作为发酵剂菌株使用的细菌菌株具备多种抵御噬菌体感染的机制：诸如包膜抗性（envelope resistance），或是可使自身完全免疫噬菌体感染的相关过程，例如限制-修饰（restriction-modification）系统与CRISPR-Cas系统。用于生产奶酪及其他乳制品的乳酸乳球菌（Lactococcus lactis），还可通过流产感染（abortive infection，Abi）阻断感染性噬菌体的增殖——该过程指被噬菌体感染的细菌会在噬菌体完成复制前发生死亡。本研究采用数学-计算机仿真模型，并结合两株乳酸乳球菌与两株裂解性噬菌体（lytic phages）开展实验，以探究流产感染（Abi）能够限制乳酸乳球菌种群内噬菌体增殖、并阻止种群被此类病毒灭绝的适用条件。根据所构建的模型，若流产感染（Abi）近乎完美生效，且不存在其他可支持该乳酸乳球菌噬菌体复制的细菌种群，则流产感染可保护细菌种群免受此类病毒感染导致的灭绝。这一预测得到了实验结果的验证：实验表明流产感染确实能够帮助乳酸乳球菌种群抵御裂解性噬菌体感染，避免种群走向灭绝。不过，研究结果同时显示，流产感染仅为乳酸乳球菌抵御噬菌体感染的防御机制之一。能够规避细菌流产感染系统的突变噬菌体会逐渐出现，此时乳酸乳球菌种群的存续便取决于能够抵御此类进化后宿主范围噬菌体（host-range phage）的包膜突变株的演化。