Penkova Elitsa, Raymond Ben
European Centre for Envronment and Human Health, Environment and Sustainability Institute, University of Exeter, Penryn, United Kingdom.
Department for Ecology and Conservation, University of Exeter, Penryn, United Kingdom.
Evol Lett. 2024 Apr 20;8(4):587-599. doi: 10.1093/evlett/qrae015. eCollection 2024 Aug.
One of the longstanding puzzles of antimicrobial resistance is why the frequency of resistance persists at intermediate levels. Theoretical explanations for the lack of fixation of resistance include cryptic costs of resistance or negative frequency-dependence but are seldom explored experimentally. -lactamases, which detoxify penicillin-related antibiotics, have well-characterized frequency-dependent dynamics driven by cheating and cooperation. However, bacterial physiology determines whether -lactamases are cooperative, and we know little about the sociality or fitness of -lactamase producers in infections. Moreover, media-based experiments constrain how we measure fitness and ignore important parameters such as infectivity and transmission among hosts. Here, we investigated the fitness effects of broad-spectrum AmpC -lactamases in in broth, biofilms, and gut infections in a model insect. We quantified frequency- and dose-dependent fitness using cefotaxime, a third-generation cephalosporin. We predicted that infection dynamics would be similar to those observed in biofilms, with social protection extending over a wide dose range. We found evidence for the sociality of -lactamases in all contexts with negative frequency-dependent selection, ensuring the persistence of wild-type bacteria, although cooperation was less prevalent in biofilms, contrary to predictions. While competitive fitness in gut infections and broth had similar dynamics, incorporating infectivity into measurements of fitness in infections significantly affected conclusions. Resistant bacteria had reduced infectivity, which limited the fitness benefits of resistance to infections challenged with low antibiotic doses and low initial frequencies of resistance. The fitness of resistant bacteria in more physiologically tolerant states (in biofilms, in infections) could be constrained by the presence of wild-type bacteria, high antibiotic doses, and limited availability of -lactamases. One conclusion is that increased tolerance of -lactams does not necessarily increase selection pressure for resistance. Overall, both cryptic fitness costs and frequency dependence curtailed the fitness benefits of resistance in this study.
抗菌耐药性长期存在的谜题之一是为何耐药频率一直维持在中等水平。关于耐药性未能固定下来的理论解释包括耐药性的隐性代价或负频率依赖性，但很少通过实验进行探究。β-内酰胺酶可使青霉素相关抗生素解毒，其具有由欺骗和合作驱动的、特征明确的频率依赖性动态变化。然而，细菌生理学决定了β-内酰胺酶是否具有合作性，而我们对感染中β-内酰胺酶产生菌的社会性或适应性了解甚少。此外，基于培养基的实验限制了我们衡量适应性的方式，且忽略了诸如宿主间的传染性和传播等重要参数。在此，我们在一种模式昆虫中研究了广谱AmpCβ-内酰胺酶在肉汤、生物膜和肠道感染中的适应性影响。我们使用第三代头孢菌素头孢噻肟定量了频率和剂量依赖性适应性。我们预测感染动态将与在生物膜中观察到的相似，社会保护作用会在很宽的剂量范围内发挥作用。我们发现，在所有情况下，β-内酰胺酶均存在负频率依赖性选择的社会性证据，这确保了野生型细菌的持续存在，尽管与预测相反，合作在生物膜中不太普遍。虽然肠道感染和肉汤中的竞争适应性具有相似的动态变化，但将传染性纳入感染适应性测量中会显著影响结论。耐药细菌的传染性降低，这限制了低抗生素剂量和低初始耐药频率挑战下耐药性对感染的适应性益处。耐药细菌在生理耐受性更强的状态（生物膜中、感染中）下的适应性可能会受到野生型细菌的存在、高抗生素剂量和β-内酰胺酶可用性有限的限制。一个结论是，对β-内酰胺类药物耐受性的增加不一定会增加耐药性的选择压力。总体而言，在本研究中，隐性适应性代价和频率依赖性都限制了耐药性的适应性益处。
