Bacterial Morphology:

“Bacteria” is a plural word. The singular for this word is “bacterium” (bacter = rod, staff). Bacteria are prokaryotes (Kingdom Monera), which means that they have no true nucleus. They do have one chromosome of double-stranded DNA in a ring. They reproduce by binary fission. Most bacteria lack or have very few internal membranes, which means that they don’t have some kinds of organelles (like mitochondria or chloroplasts). Most bacteria are benign (benign = good, friendly, kind) or beneficial, and only a few are “bad guys” or pathogens.

Kingdom Monera is a very diverse group. There are some bacteria relatives that can do photosynthesis — they don’t have chloroplasts, but their chlorophyll and other needed chemicals are built into their cell membranes. These organisms are called Cyanobacteria (cyano = blue, dark blue) or bluegreen algae, although they’re not really algae (real algae are in Kingdom Protista). Like us, some kinds of bacteria need and do best in O2, while others are poisoned/killed by it.

Bacterial Shapes
Most bacteria are one of three shapes (although there are a few other possibilities):

coccus (sing.), cocci (pl.):
are spherical (coccus = a berry),
bacillus (sing.), bacilli (pl.):
are rod-shaped (bacill(um) = a little stick), and
spirillum (sing.), spirilla (pl.):
are spiral (spiro = spiral, coil).

While many bacteria live singly, others are found in aggregates or clusters. These aggregates are named based on the arrangement of the bacterial cells of which they are composed. Using cocci as an example:

like Streptococcus pneumoniae (pneumonia) are in sets of two (diplo = double, two; pneumo = lungs), PHOTO
are in chains (strepto = bent, twisted, pliable), and
are in clusters (staphylo = a bunch of grapes).

Bacterial Cell Walls and Gram Stain:

Gram Stain
Gram Positive and Gram Negative Bacteria
Most bacteria secrete a covering for themselves which we call a cell wall, However, bacterial cell walls are a totally different thing than the cell walls we talk about plants having. Bacterial cell walls do NOT contain cellulose like plant cell walls do. Bacterial cell walls are made mostly of a chemical called peptidoglycan (made of polypeptides bonded to modified sugars), but the amount and location of the peptidoglycan are different in the two possible types of cell walls, depending on the species of bacterium. Some antibiotics, like penicillin, inhibit the formation of the chemical cross linkages needed to make peptidoglycan. These antibiotics don’t outright kill the bacteria, but just stop them from being able to make more cell wall so they can grow. That’s why antibiotics must typically be taken for ten days until the bacteria, unable to grow, die of “old age”. If a person stops taking the antibiotic sooner, any living bacteria could start making peptidoglycan, grow, and reproduce.

However, because one of the two possible types of bacterial cell walls has more peptidoglycan than the other, antibiotics like penicillin are more effective against bacteria with that type of cell wall and less effective against bacteria with less peptidoglycan in their cell walls. Thus it is important, before beginning antibiotic treatment, to determine with which of the two types of bacteria one is dealing. Dr. Hans Christian Gram, a Danish physician, invented a staining process to tell these two types of bacteria apart, and in his honor, this process is called Gram stain. In this process, the amount of peptidoglycan in the cell walls of the bacteria under study will determine how those bacteria absorb the dyes with which they are stained, thus bacterial cells can be Gram+ or Gram . Gram+ bacteria have simpler cell walls with lots of peptidoglycan, and stain a dark purple color. Gram bacteria have more complex cell walls with less peptidoglycan, thus absorb less of the purple dye used and stain a pinkish color instead. Also, Gram bacteria often incorporate toxic chemicals into their cell walls, thus tend to cause worse reactions in our bodies. Because Gram bacteria have less peptidoglycan, antibiotics like penicillin are less effective against them. As we have discussed before, taking antibiotics that don’t work can be bad for you, thus a good doctor should always have a culture done before prescribing antibiotics to make sure the person is getting something that will help.

Within the past several years, I have had several pet birds who have had digestive system or other infections. When I’ve taken them to the vet for treatment, one of the very first things she has always done is to take a sample/swab, prepare a microscope slide, Gram-stain the slide, then count and calculate the percentage of Gram+ and Gram bacteria on the slide. Birds’ systems are supposed to contain close to 100% Gram+ bacteria, so finding a high percentage of Gram bacteria is not good! In one case, I subsequently smeared some fecal material from a sick “girl” on an agar plate containing a special kind of medium called “EMB agar,” and after incubation, discovered that way that the “bad guys” in her system were all E. coli, a species that normally lives in the gut of mammals and shouldn’t be in birds. The thing is, though, if the vet can keep Gram stain dyes, microscope slides, and a microscope in the office and routinely do Gram stains, there’s no reason people-doctors can’t do the same.

Koch’s Postulates:

One “famous” person who worked with bacteria was Dr. Robert Koch, a German physician. He is famous for several discoveries related to bacteria:

    Bacteria Growing on Potato
  1. He noted bacteria growing on a spoiled potato and realized that each colony he saw grew from one bacterium that had landed on the potato. He realized he could remove a bit of one of the colonies and transfer it to a sterile medium to start a pure culture of that species of bacterium. This is called single colony isolation.
  2. Up until that time, researchers working with bacteria were trying to use gelatin to solidify nutrient media upon which to grow bacterial cultures, but there were a number of problems with this. Because gelatin is a protein, many bacteria are able to digest it and use it for food, resulting in liquification of the medium. Also, most of the bacteria that cause diseases grow best at body temperature (37° C), but as anyone who has tried taking Jell-O to a picnic knows, gelatin liquifies at that temperature. Based on a suggestion from a housewife friend, Koch developed the use of agar (a polysaccharide isolated from seaweed) to solidify nutrient media upon which to raise/grow bacteria. Because agar is a complex polysaccharide, most bacteria cannot digest it, and it remains solid at body temperature, enabling researchers to incubate bacteria, encouraging their growth.
  3. He was the first person to actually connect certain disease(s) to specific bacteria. He established four criteria, called Koch’s Postulates, which if met, prove a specific pathogen causes a specific disease (in animals):
    Koch’s Experiment
    Koch’s Postulates
    1. the same pathogen must be found in all diseased individuals (those showing the same symptoms),
    2. the pathogen must be isolated from the diseased subjects and grown in pure culture on some nutrient medium,
    3. the same disease must be induced in experimental animals by transferring bacteria from the pure culture into their bodies, and
    4. after the disease develops, the same pathogen must be isolated from the experimental animals.
    If all four of these steps can be demonstrated, then it can be said that the pathogen in question causes that disease. Koch specifically proved that anthrax and tuberculosis were caused by specific species of bacteria. Other people have demonstrated this relationship for a number of other bacterial diseases. Interestingly, this has never been done for the bacterium that causes syphilis. While we “know” what it is and how to treat it, no one has been able to grow it in culture.
Copyright © 1997 by J. Stein Carter. All rights reserved.
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