In some cases, studies of shape in model organisms under conditions mimicking natural environments have revealed selective forces that likely are relevant to pathogenic lifestyles. This has led to speculation that cell shape itself may be a virulence factor or that the mammalian host environment(s) imposes a selective pressure driving morphological diversity. Increasingly, researchers studying pathogenic bacteria in the context of infection models have observed morphological diversity and/or observed that mutants with altered colonization or virulence properties have an altered shape. Much of our understanding of the mechanisms that produce different shapes comes from the study of model organisms under laboratory growth conditions. Finally, bacteria can change morphology during their life cycle or in response to environmental conditions. Bacteria can also produce a variety of appendages, such as pili or flagella, which show diversity in overall shape, length, and width as well as placement with respect to the cell body. Bacteria have distinct cell body shapes, ranging from spheres (cocci) to rods (bacilli) of various curvatures and helicities and to more exotic shapes, such as stars, formed by elaboration of prosthecae through polar growth. We explore several aspects of morphological diversity ( Table 1). The work reviewed here builds on pioneering studies and hypotheses described in several excellent reviews on the possible selective consequences of bacterial cell shape diversity ( 1, – 5).

In this review, we consider the functional consequences of differences in form on bacterial survival in native environments (as opposed to culture flasks of rich media). Still, both pathogenic and nonpathogenic bacteria show considerable morphological diversity. Even in the research lab, cell biology approaches often focus on morphological changes to the host cell during infection, not the morphology of the pathogen itself. In the clinic, biochemical characteristics (recently including those observed by mass spectrometry) and growth on various indicator media robustly distinguish bacterial pathogens, and thus there is little incentive to directly observe bacteria. Bacteria were initially characterized on the basis of morphology, but over time, phylogenetics has proven to be a more robust method for classifying bacteria. Van Leeuwenhoek's microscopes opened our eyes to the microbial world and its abundance of forms. Still, such studies have begun to reveal the selective pressures that drive the diverse forms (or cell wall compositions) observed in mammalian pathogens and bacteria more generally, including efficient adherence to biotic and abiotic surfaces, survival under low-nutrient or stressful conditions, evasion of mammalian complement deposition, efficient dispersal through mucous barriers and tissues, and efficient nutrient acquisition. A major challenge in this field has been deconvoluting the effects of differences in the chemical properties of the cell wall and the resulting cell shape perturbations on observed fitness changes. Cell shape in most bacteria is determined by the cell wall. Here we explore shape diversity among bacteria, at the levels of cell geometry, size, and surface appendages (both placement and number), as it relates to survival in diverse environments. A greater understanding of the genetic programs underpinning morphological variation in diverse bacterial groups, combined with assays of bacteria under conditions that mimic their varied natural environments, from flowing freshwater streams to diverse human body sites, provides new opportunities to probe the functional significance of cell shape.

Many plausible models regarding the functional significance of cell morphology have emerged. Bacteria display an abundance of cellular forms and can change shape during their life cycle.

Bacteria shapes and function