Intra- and inter-specific competition and the evolution of cooperation in Bacillus thuringiensis

合作是细菌生活方式的一个共同特征。对于引起疾病的细菌(致病细菌)和与较大宿主密切相关的有益细菌(共生细菌)来说，情况可能尤其如此。许多使细菌能够利用宿主的基本工具都是基于可以被称为“公共产品”的东西。这些是细菌必须输出到细胞外的酶或毒素和其他化合物，以打破宿主细胞并获取资源。细菌还输出化学信号，将有关其在宿主内丰度的信息传达给其他细菌，从而可能协调攻击。因此，有效利用宿主需要细菌共同行动，如果低比例的细菌不能合作，细菌感染应该不那么成功，在新宿主中产生更少的感染。就资源而言，这些公共产品的制造成本很高。从进化的角度来看，合作可能是不稳定的，因为如果细菌‘欺骗’并未能为这些昂贵的合作产品做出贡献，他们可能会在宿主体内留下更多的后代。进化论对如何维持合作做出了预测。如果大多数感染是由具有类似策略的近亲确定的，代谢代价高昂的合作将使他们的亲属受益，然后他们将传播合作的基因。此外，虽然宿主内部的竞争可能导致选择作弊，但居住在不同宿主的细菌群体之间的竞争将选择更有效地利用其宿主的群体，从而进行合作。维持宿主和共生细菌之间合作的进化力量是多样的。然而，一种可能的机制是，宿主可以区分具有剥削性和非剥削性的细菌，并对不合作的共生体产生更强的免疫反应。我建议在与毒素、抗生素和化学信号的产生有关的合作方面测试这些进化的想法。初步数据还表明，Bt对寄主的利用受到与聚集性假单胞菌等共生体竞争的强烈影响。我将测试与共生体的竞争如何影响合作毒素的表达。相反，这些共生体可以与Bt合作，而不是继续作为肠道共生体与宿主合作。我将测试寄主昆虫对“作弊”共生体感染的反应。我将使用一种我熟悉的研究系统，它也具有环境和医学上的重要性。这个系统是杀虫细菌苏云金芽孢杆菌、毛虫宿主(小菜蛾幼虫)和肠道共生体聚集性泛细菌。苏云金芽孢杆菌(Bt)是一种生物农药。适用于园艺、林业、水果生产中的害虫和蚊子幼虫。它有很好的安全记录，它不会伤害人类、动物或有益的昆虫捕食者，并被批准为有机喷雾剂。虽然Bt杀虫剂在杀死害虫方面很有效，但它们在喷洒后作为一种疾病从一个害虫传播到另一个害虫的能力相对较差。改进传播将对Bt控制害虫的能力有很多好处。我实验室的初步数据表明，正如上面的理论预测的那样，合作特征对于宿主之间的有效传播至关重要。因此，了解合作如何保持有效的传输，以及传输如何保持合作，对于了解如何改进其使用至关重要。Bt与引起炭疽的细菌、炭疽芽孢杆菌和蜡状芽孢杆菌密切相关，其中几种菌株会导致人类食物中毒。这些人类病原体使用与Bt非常相似的生化机制，了解这些细菌如何合作利用宿主最终可能具有医学意义。

Cooperation is a common feature of bacterial lifestyles. This may be particularly true of bacteria that cause diseases (pathogenic bacteria) and of beneficial bacteria that live in close association with larger hosts (symbiotic bacteria). Many of the essential tools that enable bacteria to exploit hosts are based on what can be called 'public goods'. These are enzymes or toxins and other compounds that bacteria must export outside the cell in order to break open host cells and harvest the resources. Bacteria also export chemical signals that communicate information about their abundance within hosts to other bacteria and may thereby coordinate attack. Efficient use of hosts therefore requires bacteria to act collectively, if a low proportion of bacteria fail to cooperate bacterial infections should to be less successful and produce fewer infections in new hosts. These public goods are expensive to make in terms of resources. In evolutionary terms cooperation can be unstable because bacteria may leave more offspring within hosts if they 'cheat' and fail to contribute to these expensive cooperative products. Evolutionary theory has made predictions about how cooperation could be maintained. If most infections are established by close relatives with similar strategies, metabolically expensive cooperation will benefit their relative and they, in turn will then spread the genes for cooperation. In addition, while competition within hosts can lead to selection for cheating, competition between groups of bacteria inhabiting different hosts will select for groups that exploit their host more efficiently, and which therefore cooperate. The evolutionary forces that can maintain cooperation between hosts and symbiotic bacteria are diverse. However, one possible mechanism is that host can discriminate between bacteria that are exploitative or not and produce increased immune responses against symbionts that are not cooperative. I propose to test these evolutionary ideas on cooperation, in relation to the production of toxins, antibiotics and chemical signals. Prelimary data also indicate that the exploitation of hosts by Bt is strongly affect by competition with symbionts such as P. agglomerans. I will test how competition with symbionts affects the expression of cooperative toxins. Conversely, these symbionts can cooperate with Bt rather than continue to cooperate with hosts as gut symbionts. I will test how host insects react to infections with 'cheating' symbionts. I will use a study system which is familiar to me and also of environmental and medical importance. This system is the insect-killing bacteria Baccillus thuringiensis, a caterpillar host (the larvae of the diamondback moth) and the gut symbiont Pantoea agglomerans. B. thuringiensis (Bt) is used as a biological pesticide. It is applied against pests in horticulture, forestry and fruit productionan and against mosquito larvae. It has an excellent safety record, it does not harm humans, animals or beneficial insect predators and is licensed as an organic spray. While Bt pesticides are efficient at killing pests they are relatively poor at being transmitted as a disease from pest to pest after spraying. Improved transmission would have many benefits for the ability of Bt to control pests. Preliminary data in my laboratory suggests that cooperative traits are vital for efficient transmission between hosts, as the above theory predicts. An understanding of how cooperation maintains efficient transmission and transmission maintains cooperation could therefore be vital to understanding how to improve its use. Bt is closely related to the bacteria that causes anthrax, Bacillus anthracis and to Bacillus cereus, several strains of which cause food-poisoning in humans. These human pathogens use very similar biochemical machinery to Bt and a understanding of how these bacteria cooperate to exploit hosts may eventually be of medical significance.