Bacterial communities in the environment and in infections are typically diverse, yet we know little about the factors that determine interspecies interactions. Here, we apply concepts from ecological theory to understand how biotic and abiotic factors affect interaction patterns between the two opportunistic human pathogens Pseudomonas aeruginosa and Staphylococcus aureus, which often co-occur in polymicrobial infections. Specifically, we conducted a series of short- and long-term competition experiments between P. aeruginosa PAO1 (as our reference strain) and three different S. aureus strains (Cowan I, 6850, JE2) at three different starting frequencies and under three different environmental (culturing) conditions. We found that the competitive ability of P. aeruginosa strongly depended on the strain background of S. aureus, whereby P. aeruginosa dominated against Cowan I and 6850, but not against JE2. In the latter case, both species could end up as winners depending on conditions. Specifically, we observed strong frequency-dependent fitness patterns, including positive frequency dependence, where P. aeruginosa could dominate JE2 only when common, but not when rare. Finally, changes in environmental (culturing) conditions fundamentally altered the competitive balance between the two species, in a way that P. aeruginosa dominance increased when moving from shaken to static environments. Altogether, our results highlight that ecological details can have profound effects on the competitive dynamics between co-infecting pathogens, and determine whether two species can co-exist or invade each others populations from rare. Moreover, our findings might parallel certain dynamics observed in chronic polymicrobial infections.

中文翻译：

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菌株背景，物种频率和环境条件对于确定铜绿假单胞菌和金黄色葡萄球菌之间的种群动态和物种共存至关重要。

环境和感染中的细菌群落通常是多种多样的，但我们对决定种间相互作用的因素知之甚少。在这里，我们运用生态学理论的概念来了解生物和非生物因素如何影响两种机会性人类病原体铜绿假单胞菌和金黄色葡萄球菌之间的相互作用模式，这两种病原体通常在多微生物感染中同时发生。具体来说，我们在铜绿假单胞菌PAO1（作为我们的参考菌株）和三种不同的金黄色葡萄球菌之间进行了一系列短期和长期竞争实验菌株（Cowan I，6850，JE2）在三种不同的起始频率和三种不同的环境（培养）条件下。我们发现铜绿假单胞菌的竞争能力在很大程度上取决于金黄色葡萄球菌的菌株背景，其中铜绿假单胞菌主要针对Cowan I和6850，而不针对JE2。在后一种情况下，根据情况的不同，两个物种最终都可能成为赢家。具体来说，我们观察到强频率依赖性的适应性模式，包括正频率依赖性，其中铜绿假单胞菌仅在常见时才占主导，而在罕见时则不占优势。最后，环境（养殖）条件的变化从根本上改变了两个物种之间的竞争平衡，从摇动到静态环境，铜绿假单胞菌的优势度增加。总而言之，我们的结果突出表明，生态学细节可以对共感染病原体之间的竞争动态产生深远影响，并确定两种物种是否可以从稀有种群中共存或互相入侵。而且，我们的发现可能与在慢性微生物感染中观察到的某些动态相似。