Volume 44 Number 3
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 1985-1994, 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.
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.
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.
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.
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.
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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).