Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 15

Bacteria have historically been divided into two major subgroups, the Gram-negative and Gram-positive bacteria. This division was based on the response to a test called the Gram stain. “Gram-negative” bacteria retain little of the dye and are pink after this staining procedure, whereas “Gram-positive” bacteria retain more of the dye and turn deep blue. The difference in staining typically reflects the fact that Gram-negative bacteria are surrounded by a thinner structure composed of both an inner and an outer membrane, while the structure surrounding Gram-positive bacteria is much thicker, consisting of a single membrane surrounded by a thicker wall. However, this older form of classification is being replaced by talking about the phyla of bacteria as determined by the DNA sequence. The Firmicutes are a broad group containing Bacillus, clostridia, lactic acid bacteria, and the Tenericutes, including the mycoplasmas. Firmicutes have been referred to as low G+C Gram-positive bacteria based on the low percentage of guanine and cytosine (low G+C) compared to adenine and thymine often found in the genome sequence of members of this group (see chapter 1). However, having a low G+C genome is not a universal feature of the Firmicutes, which limits the utility of the designation. Another group of bacteria that were classically described as high G+C and Gram positive because they typically possess a higher percentage of guanine and cytosine includes the Actinobacteria (actinomycetes), such as Streptomyces and Mycobacterium.

The Gram designation system of classifying bacteria is particularly weak for capturing the diversity of the numerous phyla that stain Gram negative. While many bacteria historically referred to as Gram negative, such as Escherichia coli, Pseudomonas, and Rhizobium, fall within a broad group known as the Proteobacteria, many other characterized and uncharacterized groups also exist. It is also worth pointing out that relying on a staining form of classification is particularly contrived when talking about uncultured bacteria or those that are only capable of growth as symbionts in other organisms. Given all of these considerations, instead of using the Gram-positive and Gram-negative designations as a tool for describing the relatedness of groups of bacteria, we now opt for a more precise designation by referring to the phyla. In addition, while we will remind the reader of the older designations of Gram positive and Gram negative in the text when referring to the superstructure surrounding the cell, we have transitioned to describing the cell wall structure more directly.