Download The Bacteria: A quick primer

The Bacteria: A quick primer
The current thinking on the evolution of life is that the bacteria were the first cellular entities to emerge
from the primordial soup. Of course, we will never know exactly how it all played out, but this
appears to be the most likely scenario. Relative to eukaryotic cells with a nucleus and other internal
organelles, the bacteria are very simple (see microbial genetics in the archives). However, relative to
something that is not alive, the bacteria are extraordinarily complex. All things are relative.
Anton van Leeuwenhoek, a Dutch guy, got into making incremental improvements in lenses that he
was using to look at cloth (thread count has a lot to do with the quality of cloth). Well, it turns out that
counting threads was not as exciting as looking at pond water, etc, so he kept improving on the
magnification and lighting until he was able to see … bacteria. So we bestow upon Leeuwenhoek the
title, "Father of Microbiology." He spent lots of time observing, drawing, improving on his
microscopes, observing, drawing, improving on his microscopes, etc… He deserves the title.
General Structure: Bacteria have cell walls made of a rigid polymer called peptidoglycan. As the
name implies, this polymer is a carbohydrate/protein hybrid. It is found in all bacteria. The DNA is a
single circular molecule without any histones. There is also "extrachomosomal DNA" in smaller
circular molecules called plasmids. Bacterial ribosomes resemble those of Archaea and Eukarya in
appearance and function, and they read the universal genetic code in similar fashion. However, they
are uniquely bacterial. Aerobic respiration utilizes infolds in the plasma membrane to establish the allimportant proton gradient of the electron transport chain.
How small are they? The bacteria that live all over your skin, in your mouth, in your GI tract, etc…
average about one micrometer (1 µm = 1/1000 mm), compared with about 50 µm for a eukaryotic cell.
Of course, bacteria and eukaryotic cells can be smaller and larger, and there are some that overlap in
size (the largest bacteria are bigger than the smallest eukaryotic cells), but suffice to say that bacteria
are really, really small. Even the very best modern microscopes are limited in their ability to see much
more than the general shape and size of a bacterial cell….and only then if the cells are stained.
Until bacteria were grown in Petri dishes on agar-based media, they were grown in liquid broth
cultures. One cannot isolate a "pure culture" in broth, but this can be easily done on agar-based media.
Agar is a gelling agent derived from red algae. It is used in cooking, and vegans like it because gelatin
is animal protein. Yes, even blue jello is a meat product.
It is important to point out that growing bacteria in the lab limits the study of them to a tiny fraction of
all the "species" out there. Despite our ongoing efforts, less than one percent of bacterial types
(species?) have been successfully grown in culture. However, most of the disease causing bacteria can
be grown in culture. Modern molecular techniques allow scientists to identify bacteria using genetic
probes. This has changed the field of microbiology considerably.
Classification: Bacteria were lumped into a single species, Chaos infusoria, by Linnaeus, mostly
because no one really knew what they were. Were the various shapes simply a reflection of their stage
of life (like a caterpillar and a butterfly) or were they different? Once pure culture techniques were
developed, it became obvious that bacteria did not go through complex life stages, and there were lots
of types. Shape alone puts bacteria into spherical (coccus), rod-shaped (bacillus), or spiral shaped
(spirillus). The cyanobacteria form long, filamentous colonies like a very long stack of coins or beads
on a string. Others form branching filamentous colonies (the Actinobacteria). So shape became a
critical criterion for classification. Photosynthetic, chemosynthetic, heterotrophic (energy acquisition
was another criterion). Further studies and advanced microscopic techniques, respiration requirements
(oxygen required, oxygen preferred, oxygen lethal) = (obligate aerobe, facultative anaerobe, obligate
anaerobe), added yet another criterion. Then scientists developed a number of chemical tests, such as
the ability to reduce sulfur, the ability to utilize this food source or that….collectively, these types of
characteristics were used to "characterize" bacterial strains/types. There is also the ability (or lack of)
to form a very resistant structure called an endospore. Endospore-forming bacteria (Anthrax - Bacillus
anthracis, tetanus - Clostridium tetani, botulism - Clostridium botulinum) can sense when times are
tough (nothing to eat, too dry, too cold, too hot), so they wall off their DNA into a highly resistant
endospore and wait for better conditions - at which time they germinate and begin again to grow
"vegetatively." Molecular analyses are much more accurate in determining relatedness, but horizontal
gene transfer has made this as confusing as it is helpful in some cases. We know more than ever, but
this knowledge is telling us that we really don't know as much as we thought we knew before we knew
as much as we now know.
Gram Staining: Some guy named Gram came up with a staining technique for bacteria and in so
doing discovered that common bacteria fall into one of two groups - Gram positive or Gram negative.
Gram positive bacteria have a cell wall that is thicker, but there is only the plasma membrane just
inside it. Gram negative bacteria have a thinner cell wall, but they also have a second membrane
outside the cell wall (in addition to the plasma membrane). Being gram negative confers antibiotic
resistance against those antibiotics that inhibit cell wall (peptidoglycan) synthesis. This is also a
characteristic used to classify or characterize bacteria.
Monera/Prokaryote: OK, you know I'm not a fan of Kingdom Monera (Bacteria and Archaea), but
you must know that Bacteria and Archaea have been thusly assigned. Monera = Prokaryote, which
means, "not eukaryotic." The archaea and the bacteria are admittedly not eukaryotes, based on the
definition of what a eukaryote is. By the same token, amphibians are not fungi - but that doesn't tell
you much about amphibians, does it?
Reproduction: This is pretty simple. When there is ample food and moisture, and if the temperature
is OK, bacteria replicate their circular DNA molecule ("chromosome") and divide. This is called
binary fission. We've measured generation time in our lab at OHS to about 30 minutes. So, in 12
hours, a single cell can grow into a colony of millions. The world record in broth culture is around 10
minutes per generation.
Genetic exchange: Bacteria exchange DNA by way of Horizontal Gene Transfer (HGT). Check the
paper on Microbial Genetics in the archives.
Ecology: This is HUGE. Bacteria are absolutely critical for the biosphere. The fix atmospheric
nitrogen, they produce food and oxygen, they decompose dead stuff, they aid in digestion, and they do
other things that we don't fully understand. We do know with absolute certainty that without them, the
biosphere would crash and burn. Bacteria are practically everywhere. They are on and in all
multicellulars. They are in soil, water, rocks, hot springs, etc… Like the archaea, if there is an
environment with at least some water, there are bacteria.
Pathogens: A very few bacteria cause disease of multicellulars, including plants and animals (i.e.
humans). For obvious reasons, these bacteria have gotten the lion's share of attention. Many deadly
human diseases are caused by bacteria. TB, strep throat/scarlet fever, syphillus, tetanus, cholera, etc,
etc… Since Alexander Flemming discovered penicillin, these diseases are not so deadly. TB was the
#1 killer in the world a century ago. Now it's not in the top 10. But antibiotic have changed all that.
However, bacteria can achieve antibiotic resistance either by HGT or mutation, so the drug companies
try to stay one step ahead of the bacteria.