How social is social resistance?

β-lactamases are central to bacterial resistance against β-lactam antibiotics and are often treated as a textbook example of cooperative resistance. The cooperative aspect of their action arises from specific details of their action and location. Firstly, they degrade β-lactam antibiotic molecules, which detoxifies the environment for neighbouring cells as well as self, and secondly, they can act extracellularly, increasing the spatial range of this detoxifying effect and the benefit to neighbouring cells. Non-producer cells are able to take advantage of cross-protection from their cooperative neighbours and cooperator-cheat dynamics can emerge. However, mechanistic and ecological details determine the extent to which β-lactamase evolves as a result of its social action, or from private benefits to the producing cell. In the present review, we highlight recent work that reveals substantial variation in the extent to which β-lactamase-mediated resistance is shared among cells, focusing on subcellular localisation, membrane anchoring, outer-membrane vesicles, and population structure. We argue that β-lactamase spans a continuum from private to cooperative, and that recognising this variation is important both for understanding the evolution of antimicrobial resistance and for predicting treatment outcomes.