Gram-positive Bacteria

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Traditional taxonomy divides all Bacteria into two groups: Gram-positive and Gram-negative. In phylogenetic terms, this division is false. As can be seen in the tree above, the Gram-positive Bacteria do, indeed, form a coherent phylogenetic group. This single group is now commonly divided into two related phyla, the Firmicutes (a.k.a. the low G+C Gram-positive Bacteria) and the Actinobacteria (a.k.a. the high G+C Gram-positive Bacteria). However, the remaining bacterial phyla have a generally Gram-negative cell envelop. And so, like the terms “prokaryote” and “invertebrate”, the term Gram-negative tells you what an organism isn’t but not what an organism is.

Nevertheless, the use of the Gram stain as a starting point in the identification of Bacteria has been an incredibly important tool. Perhaps this is because, although the Gram-positive Bacteria represent only a fraction of the immense phylogenetic diversity of Bacteria in nature, this group contains more than its share of the most important human and animal pathogens.

The original names for the Firmicutes (low G+C Gram-positive Bacteria) and Actinobacteria (high G+C Gram-positive Bacteria) came from the fact that many of the most familiar Firmicutes have relatively low genomic G+C contents, whereas many of the familiar Actinobacteria have relatively high genomic G+C contents. This rule is by no means universal, nor particularly meaningful.

Given the incredible diversity of organisms in the phyla Firmicutes and Actinobacteria, we will only be able to touch on a few examples, focusing on less familiar representatives rather than the well-known pathogens.

What does being Gram-positive mean?

The structural distinction between Gram-positive and Gram-negative Bacteria lies in the structure of the cell envelop. Gram-positive Bacteria have no phospholipid outer membrane; they are bound by the cytoplasmic membrane and usually a very thick cell wall (& so stain Gram-positive). There is therefore no periplasmic space, that Gram-negative Bacteria use as an environmental buffer. But they do have some control of the conditions in the spongy thick wall, and so this may serve a purpose similar to that of the periplasmic space. Other Bacteria (Gram-negative Bacteria) have both an outer & inner membrane, sandwiching a thin cell wall, and therefore stain Gram-negative. Like many other Bacteria and Archaea, some Gram-positive Bacteria also have an outer protein coat called the S-layer.

However, a wide range of factors, in addition to the outer membrane and cell wall thickness, determine whether or not a culture stains Gram-positive or negative. Even in the same species, cells may stain differently in different stages of the growth cycle or when grown under different conditions.

In addition, some members of the Gram-positive phylogenetic group do have an outer membrane. Some Actinobacteria have an outer membrane composed of mycolic acids rather than phospholipids. These organisms are sometimes considered neither Gram-positive nor Gram-negative, but “acid fast”. Some Firmicutes have a traditional Gram-negative envelop, complete with phospholipid outer membrane and lipopolysaccharide. It is very important, therefore, when using the terms “Gram-positive” or “Gram-negative”, to be clear about whether this refers to how the cells stain, the structure of their envelop, or a phylogenetic group.

An alternative view of Gram-positive Bacteria

Some molecular phylogenists argue, primarily on the basis of conserved insertions and deletions (indels) in protein-coding genes, that the Gram-positive Bacteria are more closely related to Archaea and eukaryotes than are any other group of Bacteria. This view is not supported by analyses of conserved protein or RNA gene sequences, but it does separate organisms into those with 2-membrane envelops (Gram-negative Bacteria), which in this scheme are termed “Diderms”, and those with single-membrane envelops (Gram-positive Bacteria, Archaea and eukaryotes), termed “Monoderms”. This issue remains a matter of contention.