Shigella is a genus group of bacteria that contains four species, S. dysenteriae, S. flexneri, S. boydii, and S. sonnei.  While these are frequently referred to as “species,” it is probably more scientifically correct to refer to them as biotypes of Escherichia, but because of the important medical differences between infections with Shigella versus most Escherichia the naming convention remains.  As is also true of Salmonella species, there are numerous serotypes found within the different Shigella species, with the notable exception of S. sonnei which has only one serotype.  Serotyping is important for scientific, especially epidemiological, studies, but is not likely to be relevant to most people.Shigella 1

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Serratia is the genus name of a group of relatively common bacteria, the most common of which is Serratia marcescens.  There are other species within the genus, a few of which have on rare occasion caused disease in humans, but the vast majority of infections with the Serratia occur with Serratia marcescens, and from this point forward, any mention of Serratia is understood to refer specifically to S. marcescensSerratia 3

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Salmonella is the genus name of a large group of bacteria which however contains only two distinct species.  Unlike many bacterial genus groups the distinctions among the many, at last count 2,501, individually recognized types of Salmonella are not species but instead are known as serovars, or serological variants.  These variants are recognized by differing antigens expressed on the surface of the bacteria instead of the variations in DNA sequence that mark different species.  Serovars are generally named for the geographic location of their first isolation.  The differing serovars, with a few important exceptions, are not relevant to the vast majority of people as they rarely change the course of the infection and subsequent disease, again, there are several important exceptions, but they are of critical value to epidemiologists and others who track and combat the large scale outbreak forms of these infections which occur in the United States on a yearly basis.Salmonella 1

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Pseudomonas is the genus name of a large group of bacteria, some of which can infect and sicken humans.  The number of bacteria assigned to Pseudomonas has declined in recent years as the advent and widespread adoption of genetic testing of bacteria has caused many former members of Pseudomonas to be moved to existing, or in many cases completely new, genus groups.  Of the remaining members of the genus Pseudomonas, only a few have been implicated in human illnesses and of those by far the most common culprit is Pseudomonas aeruginosa.  From this point forward, for our purposes, any mention of Pseudomonas is understood to refer specifically to P. aeruginosa.PA 2

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Listeria monocytogenes is one specific genus and species pair of bacteria.  There are five additional species within the Listeria genus, but only one of these, L. ivanovii, in addition to L. monocytogenes, is considered to be a pathogen, or capable of causing disease.  L. ivanovii is not nearly as common an infection however and it is nowhere near as great a concern as L. monocytogenes, which is an organism of great public health concern as it often occurs in large scale outbreaks, sometimes spanning multiple states, due to its association with contaminated fresh and processed food.  Because most all references to Listeria are linked specifically to L. monocytogenes, from this point forward this specific organism will be referred to by the shorthand of Listeria.

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Legionella is the genus name of a large group of bacteria capable of causing, potentially severe and even fatal, disease in humans. While there are now over 40 named species and some 60 additional serogroups within those species, only about half of them have been implicated in human disease, the most common cause being Legionella pneumophilia, although L. longbeachae, L. dumoffii, and L. bozemanii are the other most commonly isolated species causing human illness. The balance are primarily environmental contaminants, especially of water and water containing systems. The most common source of infection are cooling towers used for air conditioning systems, especially in commercial settings, although swimming pools, showers, sinks, drinking fountains, hot tubs, mist machines, ice machines, and even decorative fountains have also been implicated as infection sources. Infection occurs when the bacteria is inhaled with water vapor or mist, although most people who are exposed and infected do not become ill. There is no risk of person to person transmission.

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Klebsiella pneumoniae is the specific name of a bacterium that is capable of causing serious illness, especially pneumonia, as the name suggests, in susceptible people.  Klebsiella bacteria are found in several areas of the human body, especially in the intestinal tract, where they do not cause disease, but when circumstances allow, they can be very damaging, causing a range of infections, including, but not limited to: pneumonia (as mentioned), urinary tract infections, septicemia, meningitis, and soft tissue infections. Unfortunately, some strains K. pneumoniae are quite virulent and can even prove fatal, but this almost always happens in patients already debilitated due to other causes, and fortunately these extreme outcomes are rare.KP 1

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Escherichia Basics

Escherichia is the genus name of a group of bacteria that contains at least 7 different species, although by far the most well-known and well-studied is clearly Escherichia coli (E. coli).  Because E. coli is so common in the environment and in humans, as well as in other animals and because it is so easy to propagate (under the right conditions it will reproduce in as little as 20 minutes) is among the most well understood and manipulated of all the bacteria.  When speaking of E. coli, one is speaking in generalities since there are hundreds of strains of E. coli which vary significantly, but not significantly enough for them to be considered separate species.  The differences among the strains are important because while most E. coli are harmless, provided they remain in their normal environment of the gastrointestinal tract, some strains are quite virulent and can even prove fatal, even in otherwise healthy individuals.  Fortunately, these extreme outcomes are rare.

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Enterococcus is the genus name of a group of bacteria that contains about 20 different species that have been implicated in human disease at some point.  However, the overwhelming majority of such infections result from Enterococcus faecalis (E. faecalis) with most all of the remaining infections being accounted for by Enterococcus faecium (E. faecium).  The other Enterococcus species (Enterococcus sp) are rarely implicated in human disease and there are several other Enterococcus sp that are only found in the environment.  All of these bacteria are quite hardy and can live on environmental surfaces for extended periods.E faecalis 3

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