Download Bacteria 101

March 2002: Volume 44, Number 3
1.
Bacteria 101
by Kelly A. Reynolds, MSPH, Ph.D.
Despite the great strides in water treatment to eliminate bacterial pathogens, these agents continue to contribute to waterborne
disease outbreaks (see Table 1). Of the 21 documented waterborne bacterial disease outbreaks in the United States from 19851994, 12 were due to Shigella, six to Campylobacter, two to Salmonella and one to Escherichia coli O157:H7. Drinking water
sources may contain a wide variety of bacteria that are opportunistic or overt pathogens in addition to over 80 genera of
nonpathogenic bacteria.
Since the development of vaccines and antibiotic treatments, bacteria had not been as great a public health threat as they were in
past centuries. In recent decades, the emergence of new, more virulent strains and bacteria that are resistant to many antibiotics
have caused scientists to re-examine these microorganisms with increased respect.
Bacterial morphology
Bacteria are the smallest (0.5 to 1 ?m in diameter), simplest, single-celled organisms (known as prokaryotes) that can live
completely on their own with all the biochemical equipment needed for metabolism. Bacteria consist primarily of a cellular
material contained within a cell wall. They’re classified largely on the properties of the cell wall, the most common distinction
being Gram positive (having a thick, single layered cell wall) or Gram negative (a multi-layered cell wall with an outer
membrane).
Bacteria may be shaped like spheres (cocci), spirals (helical) or cylinders (rods). Those with no defined shape are called
pleomorphic. Rod-shaped bacteria may have external appendages (flagella) to aid in their motility. Cells usually exist in chains or
aggregates, known as colonies.
All bacteria require water, nutrients and a source of energy for growth. Oligotrophic bacteria can live in nutrient-deficient
environments, whereas copiotrophic bacteria survive only in nutrient-rich environments. Other terms to be familiar with relative
to bacteria are listed in Table 2.
Capable of multiplying every 20 minutes, bacteria can quickly increase to large numbers in favorable environments. Bacterial
cells have been found surviving at great extremes, such as temperatures ranging from 482oF to below freezing. Some have few
nutritional needs and can grow in distilled water or even harsh chemicals. Perhaps the most amazing trait of bacteria is their
ability to adapt and survive under just about any environmental conditions; hence, the increasing problem of bacterial resistance
to disinfectants.
Pathogenic bacteria
Most bacteria are harmless to humans and have been used to produce medicines, foods and functional enzymes. In addition,
bacteria are essential for the degradation and nutrient cycling of plants. Some bacteria, known as pathogens, cause disease in
humans. Of particular concern to the water industry is the enteric bacteria, which are able to colonize the human gut and cause a
wide variety of diseases following ingestion of the organism. Enteric bacterial pathogens typically wreak havoc by either
competing with normal intestinal cells for nutrients or by secreting toxins that cause a variety of mild to severe symptoms. Other
water-related transmission routes include skin exposures, causing rashes and irritations, and respiratory exposures from shower
and fountain aerosols.
Lipopolysaccharide (LPS), or endotoxin, is usually the pathogenic component of Gram negative cells. Endotoxins comprise the
outer layer of the cells, are very heat stable, and induce a strong immune response in hosts when the bacteria die and the cell wall
is broken down. Most enteric illnesses are caused by this group of bacteria. Yersenia pestis (agent of the Black Plague) is an
example of a Gram negative rod.
Other bacteria produce exotoxin, a harmful protein that’s released outside the cell during growth. Cholera, botulism and diptheria
are all examples of exotoxins. Enterohemorhagic and enterotoxigenic E. coli also produce exotoxins. Shigella dysenteriae
produces both an endotoxin and an exotoxin.
Indicator bacteria
Indicator organisms are used in water microbiology as evidence that fecal contamination has occurred. Indicators are chosen
because they’re easier to detect than specific pathogens. They may include fecal and total coliforms (first used in 1880), fecal
streptococci, clostridia and others. No organism, however, serves as a universal indicator of all pathogenic species. For example,
pathogenic bacteria that do not correlate with coliform indicators include Pseudomonas, Aeromonas, Plesiomonas, Yersinia,
Vibrio, Legionella and Mycobacterium. This is particularly worrisome as the monitoring of indicator species alone may not
protect the health of the public against other harmful agents.
Opportunistic bacteria
Some bacteria are pathogens only under specific conditions, such as when a severely immunocompromised person is exposed.
These bacteria may be natural inhabitants of water such as Pseudomonas, Serratia, Acinetobacter, Chromobacterium,
Achromobacter, Aeromonas, etc. Or, they may be introduced from surrounding soil and plants such as Bacillus, Enterobacter,
Klebsiella, Actinomyces, Streptomyces, etc. Opportunistic bacteria may also be introduced to water from regrowth and biofilm
conditions in the distribution system.
Nuisance bacteria
A group of bacteria known as iron bacteria (i.e., Gallionella and Siderocapsa) and sulfur bacteria (i.e., Desulfovibrio and
Thiobacillis) include organisms capable of depositing large amounts of iron or sulfur, and associated slimes, during their
lifecycle. These deposits may foul or plug pipes, cause odor, taste and color changes in water, and increase the turbidity and
corrosiveness of the water.
Earthy-musty odors are among the most difficult natural odors to eliminate from municipal water supplies in many regions
worldwide. Although originally attributed to chemical contaminants, it’s known today that these odors are due to microbial
agents called Actinomycetes. Resembling fungi in appearance, Actinomycetes are actually filamentous, branching bacteria. The
most common known to water supply problems is Streptomyces.
Stressed organisms
Indicator bacteria, such as fecal and total coliforms and fecal streptococci, become stressed or injured in water, causing them to
enter a non-reproductive state. Due to this condition, cultural analysis may fail to detect anywhere from 10-to-90 percent of the
indicator population, leading to false negative findings that could impact water quality and public health. Common stress
conditions include disinfection, the presence of metals and salts, extreme temperature or pH, solar irradiation and many others.
Under conditions of limited nutrient supply, certain gram-positive rods can form a highly resistant, environmentally stable,
dehydrated form called spores. Clostridium botulinum (causative agent of botulism) and Bacillis anthracis (causative agent of
anthrax) are two examples of spore-forming bacteria. Although highly resistant to temperature extremes, bacterial spores are
effectively disinfected with chlorine bleach.
Conclusion
Pathogenic bacteria continue to be recognized. The four bacteria listed on the U.S. Environmental Protection Agency’s
“Contaminant Candidate List” are Aeromonas, Cyanobacteria, Helicobacter and Mycobacterium avium complex. Experts agree
that there’s a sparse database on these organisms with little information on occurrence, exposure and risk assessment. It’s
important to understand the basics of bacteria to develop products to minimize the impact of these agents on public health. With
the emergence of poorly understood bacteria, it’s increasingly important to prevent bacterial infections before they occur rather
than rely on post-infection treatment.
About the author
Dr. Kelly A. Reynolds is a research scientist at the University of Arizona with a focus on development of rapid methods for
detecting human pathogenic viruses in drinking water. She holds a master of science degree in public health (MSPH) from the
University of South Florida and doctorate in microbiology from the University of Arizona. Reynolds also has been a member of
the WC&P Technical Review Committee since 1997.
Table 1: Pathogenic bacteria transmitted by drinking water
Vibrio cholera -- Potentially fatal infections. Rare in the U.S.; usually associated with inadequate sanitation practices.
Salmonella spp. -- Ubiquitous in the environment but in low numbers.
Shigella spp. -- Most common in food outbreaks and with children. Poor survival in the environment, easily killed by
disinfection methods.
Toxigenic Escherichia coli -- May be fatal, especially in children and elderly hosts. Low infectious dose.
Campylobacter spp. -- Normal flora of many wild and domestic animals (birds, sheep, goats, chickens).
Leptospira spp. -- Widespread disease of wild and domestic animals- incidental human hosts. Rare in temperate regions. May be
fatal.
Francisella tularensis --One of the most infectious bacteria known. Causes tularemia. Spread from animals. Potential biological
warfare agent with high mortality; resistant.
Yersenia enterocolitica -- Hardy, cold-water organism, common to wild and domestic animals with water habitats. Sporadic U.S.
cases.
Aeromonas spp. -- Natural aquatic organisms. Proliferate in warmer months. Economic threat to aquaculture. Documented
association with gastric illness, no U.S. outbreaks.
Helicobacter pylori -- Primary cause of stomach ulcers. Infects 90% of persons in developing countries and up to 60% of persons
in developed nations.
Legionella pneumophila -- Long-term survival in specific environments, i.e., cooling towers, distribution systems. Legionnaire’s
disease, Pontiac fever- infections can be mild, severe or fatal.
Mycobacterium avium complex -- Complex cell wall infers increased resistance to disinfectants. Widespread in nature. Causes
chronic pulmonary disease in immunocompetent hosts and disseminated disease in immunocompromised hosts.
Table 2: Terms of bacterial growth and metabolism
Autotrophic bacteria -- Important in the cycling of inorganic nutrients, methane production and conversion of ammonia to
nitrate. Derive energy and carbon from inorganic sources. Carbon obtained from carbon dioxide.
Photoautotroph -- Energy derived from sunlight.
Chemoautotroph -- Energy derived from the oxidation of inorganic substances.
Heterotrophic bacteria -- Derive carbon from preformed organic compounds that are broken down enzymatically.
Photoheterotroph -- Derive energy from light (i.e., green and purple sulfur bacteria).
Chemoheterotroph -- Energy is derived through oxidation of organic compounds via respiration (generation of energy through
chemical oxidation). Most pathogens are chemoheterotrophic.
Aerobic bacteria -- Utilize oxygen.
Anaerobic bacteria -- Utilize combined forms of oxygen such as carbon dioxide, nitrate, sulfate, or a metal such as iron. Oxygen
is lethal.
Facultative anaerobes -- Prefer to utilize oxygen but can use other substrates.
Mesophiles -- Grow best at 20-45?C (human body temperature=37?C).
Psychrophiles -- Predominantly pseudomonads, grow best at refrigeration and freezing temperatures (0-20?C).
Thermophiles -- Grow best at 45-60?C and up to 90?C.
Halophiles -- Tolerant to salt concentrations up to 30% (near saturation).