The World of bacteria I: Ordinary Gram-Negative Bacteria

The World of bacteria I: Ordinary Gram-Negative Bacteria

The most widely used reference for bacterial classification is Bergey’s Manual of systematic bacteriology, now published in four volumes. Volume 1 includes mainly the familiar or ordinary Gram-negative chemoheterotrophic eubacteria, many of which have clinical, industrial, or agricultural importance. Most of the organisms have a relatively simple morphology and cellular arrangement. They do not form complex structures such as prostheses (semirigid extensions of the cell wall and cytoplasmic membrane), sheaths (hollow tubes that enclose chains or trichomes), or, with rare exceptions, endospores (heat-resistant, thick-walled refractive forms) and cysts (thick-walled forms desiccation-resistant but not heat-resistant). The cells are not arranged in tri-chromes. They reproduce mainly by transverse binary fission rather than by budding, fragmentation, or spore formation. Motility, if present, is of the free-swimming type rather than the gliding type. The organisms are mainly heterotrophic, but some can grow autotrophically in the presence of H₂. Some are saprophytes (live on dead organic matter). Others are parasites (living in association with a host). Some are highly pathogenic (often cause disease), others may be opportunistic pathogens (cause disease only in a patient whose defense mechanisms against infection have been weakened or compromised), and others are not known to cause disease.

The spirochetes

The distinguishing features of spirochetes are:

1. A helical shape
2. An ability to twist or contort their shape (i.e., flexibility)
3. The occurrence of a special kind of flagella termed periplasmic flagella (also called axial fibrils or endo flagella)

The major difference in structure from other gram-negative eubacteria is in the location of the periplasmic flagella. They are located between the outer membrane (often termed the outer sheath in spirochetes) and the protoplasmic cylinder (i.e., the protoplast plus the overlying peptidoglycan layer); thus they are located in the periplasmic space of the cell. The periplasmic flagella have an ultrastructure similar to that of ordinary flagella, including the abasal body with disks, and they are responsible for the swimming motility of spirochetes; however, since periplasmic flagella do not extend outward from the accomplish this motility is not yet clear. Spirochetes swim best in media of high viscosity, whereas bacteria with ordinary flagella swim best in media of low viscosity; spirochetes can also exhibit a creeping or crawling motility when in contract with solid surfaces. Most spirochetes are so thin that they cannot be easily seen by light microscopy, even when gram-stained; however, dark-field microscopy does provide sufficient contrast and is the method of choice for visualizing these organisms.

The spirochetes comprise the order Spirochaetales. This order is divided into two families, which are distinguished as follows:

The family Spirocheataceae

1. They are strongest anaerobes, facultative anaerobes, or microaerophiles.
2. Carbohydrates or amino acids are used as carbon and energy sources.

The family Leptospiracle

1. They are aerobes
2. Long-chain fatty acids are used as the carbon and energy source.

Aerobic/Microaerophilic, Motile helical/Vibrioid, Gram-negative bacteria

These bacteria have the following characteristics:

1. The cells are rigid (unlike spirochetes) are range from vibrioid (having less than one turn or twist) to helical (having one too many turns or twists).
2. They swim by means of polar flagella.
3. They are aerobic or microaerophilic.
4. The organisms usually give a positive reaction by the oxidase test (a laboratory test based on the presence of cytochrome c.)

Most of the organisms are harmless saprophytes and occur in freshwater or marine environments, but a few are parasitic nd can be pathogenic for humans and animals or for other bacteria. Some examples of genera in this section of Bergey’s manual are as follows:

Aquaspirillum

Aquaspirillum is helical or vibrioid organisms that typically possess bipolar tufts of flagella. These harmless saprophytes are anaerobic to microaerophilic and occur in stagnant stream or pond water. No growth occurs in the presence of 3% NaCl or sea water.

Azospirillum

The cells are plump and vibrioid with a single polar flagellum and if grown on solid media, with numerous lateral flagella as well. Azospirilla occurs within the roots of grasses, wheat, corn, and many other plants or as free-living soil organisms. They fix N₂ within plant roots or in laboratory cultures. Under N₂-fixing conditions they are microaerophilic, but they are aerobic if supplied with a source of fixed nitrogen such as ammonium salt. One species, A.lipoferum, can grow autotrophically with hydrogen gas as the energy source.

Oceanospirillum

The cells are helical, usually with bipolar tufts of flagella. Oceanospirilla is aerobic and is harmless saprophytes, occurring in coastal marine waters. Sea water is required for their growth.

Campylobacter

These Vibiroid cells have a single flagellum at one or both poles. Campylobacters are microaerophilic parasites, occurring in the reproductive organs, intestinal tract, and oral cavity of humans and other mammals. Some species are pathogenic, e.g., C.jejuni which causes diarrhea in humans, or C.fetus subspecies Venerealis which causes abortion in cattle.

Bdellovibrio

These aerobic, vibrioid cells possess a single polar flagellum. Bdellovibrio has the unique property of being parasitic on other gram-negative bacteria. After attachment to a host bacterium, the Ddellovibrio penetrates the outer membrane of the cell wall and grows within the periplasmic space. Eventually, the host bacterium becomes an empty “ghost cell”. Bdellovibrio occurs in soil sewage and in freshwater ad marine environment. The genus Vampirovibrio has certain similarities to the genus Ddellovibrio, but the organisms attack eukaryotic algae, not bacteria.

Nonmotile , gram-negative, curved bacteria

The characteristics of the bacteria in this section are as follows:

1. Rigidcells that are curved to various degrees, forming coils, helical spirals, and sometimes rings (i.e., cells that are curved around so that the ends overlap).
2. Nonmotile

These harmless saprophytes occur mainly in soil, freshwater, and marine environments. One example is the family Spirosomaceae, which contains three genera whose cells, which are aerobic, form no intracellular gas vacuoles, are catalase and oxidase positive, and form colonies that are yellow (genus Spirosoma) or pink (the Genera Runella and Flectobacillus). Another common genus, not included in the family Spirosomaceae, is Microcyclus, which forms prominent intracellular gas vacuoles and whose colonies have no pigment.

References

Arvind, Keshari K. and Kamal K Adhikari. A Textbook of Biology. Vidyarthi Pustak Bhander.

Michael J.Pleczar JR, Chan E.C.S. and Noel R. Krieg. Microbiology. Tata Mc GrawHill, 1993.

Powar. and Daginawala. General Microbiology.

Rangaswami and Bagyaraj D.J. Agricultural Microbiology.