New Methods in Electron Microscopy Help Elucidate the Structure of the Murein Sacculus and the Distribution of Penicillin-Binding Proteins

Abstract

Since bacteria are so small (usually about 1.5–2.5 μm³), microbiologists have relied on microscopy to differentiate single bacteria from their neighbours, to determine cellular shape and form, to distinguish gram-negative cultures from gram-positive ones, and to detect the position of envelope adornments (e.g., flagella and capsules). These characteristics could usually be determined by light microscopy and it was not until the early 1950s that more detail was discovered by electron microscopy (Chapman and Hillier, 1953). The absence of a nucleus was confirmed, and the unequivocal detection of cell wall layers, and their contribution to cell division, outside a bilayered plasma membrane was established (Murray et al, 1965). These were halcyon days for microbial structure since electron microscopy was capable of discerning most of the structural bits and pieces which composed intact bacterial cells (Holt and Beveridge, 1982). Ribosomes and chromosomal DNA fibres could be discerned within the cytoplasm, and gram-positive and gram-negative bacteria could be distinguished by their cell envelope profiles. The former possessed an amorphous, electron dense wall above the plasma membrane, whereas the later possessed a more complex profile consisting of a thin peptidoglycan layer sandwiched between a plasma membrane and an outer membrane (Beveridge, 1981). Indeed, these early observations of envelope profiles by thin section firmly established the existence of a periplasmic space (i.e., a region encompassing or surrounding the cellular plasm) and, later, of Bayer’s so-called “adhesion zones” (Bayer, 1974) in gram-negative bacteria.