Bacteria are generally single cell organisms. Typically, they are 0.3–1.0 microns (1000th of a millimetre) wide by 0.5–8 microns long. Bacteria have a thin cell wall that surrounds a uniform protoplast composed mostly of gelatinous cytoplasm and RNA and DNA.
Bacteria that have the potential to cause infection are referred to a ‘pathogenic.’
Shape and Movement
Spherical bacteria are called cocci and those with a rod shape are known as bacilli. There is tremendous variation in size and shape within each group.
Some bacteria have thin whip-like flagella (tails) for locomotion, while others move by means of hair-like filaments protruding from their walls called pili, which may also help the bacteria attach themselves to potential host cells.
Bacteria reproduce through binary fission. During this process a cell enlarges and then divides into two identical organisms. Binary fission allows for tremendous growth of a bacterial colony.
For example, under ideal conditions, E. coli divide every 20 minutes. Thus 1,000 organisms become 2,000 and 20 minutes later 4,000 and so on in a doubling process. Within 24 hours one E. coli will theoretically produce 4.74 x 1021 organisms!
Fortunately, such growth rarely occurs in nature because of insufficient nutrients to support unbridled reproduction, temperature swings, unfavourable pH and predatory microorganisms. Not surprisingly, the human intestine is an ideal environment for bacterial reproduction and human infection is invariably via the faecal–oral route.
When faced with adverse environmental conditions, such as a lack of nutrients, some bacteria turn into spores as a survival mechanism. During this process, each bacterium creates a tough round outer shell to protect the vital RNA and DNA components. Once transformed into a dehydrated spore, the bacterium is in a dormant state and cannot reproduce.
Spores can survive for hundreds of years and only reactivate when introduced into a favourable environment. While many spore-forming bacteria are pathogenic, such as C. tetani, which causes tetanus after entering an open wound, spores do not seem capable of causing infection if ingested, and no waterborne-pathogenic bacteria produce spores.
Taxonomy, Nomenclature and Classification
Of the thousands of types of bacteria catalogued by microbiologists, few are actually human pathogens. Microbiologists use several, somewhat confusing, systems to classify bacteria.
The Linnaean hierarchical taxonomic system, which classifies organisms into families, genera and species with Latin names, is insufficient. Bacteria are not studied as individual organisms but rather as ‘pure cultures’ composed of individuals derived from a hypothetical single source organism. These individuals are no longer necessarily identical because of genetic mutation or changes in response to varied environments.
Microbiologists apply the term ‘strain’ to a group of pure cultures derived from either a common source organism, several organisms within one infected person or different organisms from various people in a common source outbreak. Similar strains are then grouped into a species. However, species boundaries, normally defined by the limits of cross-fertility, do not apply to bacteria which reproduce asexually and freely exchange genetic material.
Despite problems with the Linnaean system, in 1980, the International Committee of Systematic Bacteriology (ICSB) published the definitive list of 2,500 species of bacteria to replace a list that had mushroomed to over 30,000 species.
Currently, only the names from the ICSB list, which are Linnaean in origin, are considered valid, and any additions or changes require publication in the International Journal of Systematic Bacteriology.
In the Linnaean system, the first Latin name is capitalized and refers to the genus, while the second Latin name is in lower case letters and identifies the species. For example, in the case of Salmonella typhi, Salmonella identifies the genus and typhi the species. It is customary to abbreviate the singular genus name with a capital letter followed by the species name in lower case, i.e. S. typhi.
Bacteria are also classified based on characteristics.
In 1923, David H. Bergey developed the most widely used classification system for the American Society for Microbiology (ASM). The most recent edition is Bergey’s Manual of Systematic Bacteriology published in five volumes between 2001 and 2011.
Bergey’s classification system groups bacteria into ‘sections’ according to easily identifiable properties. The sections cut across taxonomic boundaries, have no formal taxonomic status and are purely for ease of use.
There are 33 sections, each with its own list of group properties. These include cell wall composition, shape, aerobic or anaerobic status and whether the bacteria are spore producing. With few exceptions, waterborne bacteria come from section two—aerobic and microaerophilic, motile, helical and vibrioid, Gram-negative bacteria—and most often section five—facultatively anaerobic, Gram-negative, rods.
Section five has three families of bacteria: the enterics, the vibrios, which are curved rods and the pasteurellas. Only the enterics and vibrios are important as waterborne pathogens. Gram-negative or positive refers to how bacteria cell walls react to Gram’s method of differential staining performed to enhance viewing under a microscope.
When microbiologists talk about bacteria they usually call them by their formal Linnaean terms for genera and species but describe them using Bergey’s scheme. For example, bacteria in the genera Salmonella are often referred to as Gram-negative, flagellated, non-sporulating, aerobic, bacilli. This means that they give up violet dye in Gram’s test and must be stained red to be seen, move by means of a whip-like flagella (tail), do not form spores, use oxygen and are rod shaped.
Waterborne Pathogenic Bacteria
The following tables summarize common and uncommon but important waterborne pathogenic bacteria found in North America and Europe, their associated illness and treatment.
|Campylobacter jejuni||Gastroenteritis, dysentery, possible extra-intestinal complications||Fluid and electrolyte replacement and anti-microbial drugs|
|Salmonella species||Gastroenteritis, small intestine dysentery, enteric fever, possible extra-intestinal complications||Fluid and electrolyte replacement and anti-microbial drugs|
|Shigella species||Shigellosis (bacillary dysentery) and possible extra-intestinal complications||Fluid and electrolyte replacement and anti-microbial drugs|
|Escherichia coli||Gastroenteritis, shigellosis-like bacillary dysentery||Fluid and electrolyte replacement and anti-microbial drugs|
|Y. enterocolitica||Gastroenteritis, shigellosis-like bacillary dysentery||Fluid and electrolyte replacement and anti-microbial drugs|
|Vibrio cholerae||Cholera characterized by severe dehydration often resulting in death||Massive oral and / or intravenous fluid and electrolyte replacement|
|Aeromonas||Asymptomatic excretion, gastroenteritis, and possibly dysentery||Fluid and electrolyte replacement|
|Plesiomonas||Gastroenteritis, severe dehydration, and fever||Fluid and electrolyte replacement|
|Leptospira interrogans||Asymptomatic excretion, flu-like illness, and Weil’s disease||Immediate drug therapy, and fluid and electrolyte replacement|
|Legionella pneumophila||Pontiac fever and Legionnaire’s disease||Anti-microbial drugs|
|Listeria monocytogenes||Listeriosis, spontaneous abortion or still birth in pregnant women||Anti-microbial drugs|
Most waterborne pathogenic bacteria are also foodborne pathogens. Invariably, the transmission of waterborne pathogenic bacteria is by the faecal–oral route, meaning that bacteria are ingested in water contaminated with human or animal faeces. The amount of bacteria shed per gram of faeces from an infected person or animal varies according to the severity of infection and the type of bacteria.
The infective dose is the minimum number of pathogenic bacteria that can cause an infection in humans. It varies widely between organisms from as few as ten to several million. The infective dose is lower when consumed in water than food because water passes through the acid bath of the stomach faster, so many bacteria make it into the small intestine alive.
In the case of almost all pathogenic bacteria, the severity of infection is directly related to how many organisms are eaten. Low doses may cause asymptomatic infection, while heavy doses cause the worst illness.
Humans and animals are the natural habitat or reservoirs for pathogenic bacteria. Abundant nutrients, an optimum temperature, and good pH make our intestines the ideal bacterial breeding grounds.
While many pathogenic bacteria can multiply on food, few can multiply in water because of low nutrient content and variable temperature and pH. Consequently, water is a natural reservoir for only a few bacteria including Vibrio cholerae, which causes cholera, and Salmonella typhi, which causes typhoid fever.
This is good news because when bacterially-contaminated faeces get into water sources, the number of organisms begins to decline almost immediately. Dilution, UV radiation, water temperature, pH and predatory microorganisms all help to disperse or kill bacteria in untreated water.
Another advantage is that because humans are the only natural reservoirs for some bacterial pathogens, unless infected humans are constantly depositing faeces in the water the bacteria will eventually die off.