There are many Gram-negative bacilli of medical significance. The most important of these are members of the family Enterobacteriaceae. Other genera of medical importance include Vibrio, Campylobacter and Pseudomonas.
The family Enterobacteriaceae is the largest and most heterogeneous collection of medically important gram-negative bacilli and are commonly isolated from clinical specimens. There are over fourteen genera that been described to cause human infection but by far the most important single species is Escherichia coli. Salmonella are a major cause of gastroenteritis. Other important genera include Shigella, Proteus, Klebsiella, Enterobacter, Citrobacter, Serratia and Yersinia.
Enterobacteriaceae are are found in soil, water, and vegetation and are part of the normal enteric flora of all animals including humans. Some members of the family e.g. Shigella, Salmonella, Yersinia pestis (the plague bacillus) are almost always associated with disease when isolated from humans i.e., they are never “normal” flora, whereas others e.g. Escherichia coli, Klebsiella pneumoniae, and Proteus mirabilis are members of the normal commensal flora and usually cause opportunistic infections.
Infections caused by the Enterobacteriaceae can originate from an animal reservoir (e.g. most salmonella infections), from human’s with illness (e.g. Shigella spp., and Salmonella typhi) or by endogenous spread of organisms in a susceptible patient (e.g. E. coli urinary tract infections). Infections can involve virtually any body site. More than 5% of hospitalized patients develop hospital acquired (nosocomial) infections, with members of the Enterobacteriaceae responsible for a large proportion of these infections.
Members of this family are moderate-sized, non spore-forming Gram-negative bacilli, and are either motile by means of peritrichous flagella or are non-motile. All members grow both aerobically and anaerobically (referred to as being “facultatively anaerobic”) with growth observed after 18 – 24 hours of incubation on a variety of non-selective and selective media. They are generally not difficult to cultivate and have simple nutritional requirements. They all have the ability to ferment glucose, and reduce nitrates to nitrites. The latter characteristic is utilized in chemical analysis of urine as a screening test for their presence.
Selective and differential media, biochemical tests and serological methods are used to identify these organisms.
The cell envelope of Gram negative bacteria have a thin peptidoglycan layer sandwiched between two membranes. The space between the two membranes is referred to as the periplasmic space. The inner membrane is the cell membrane common to all bacteria and the outer membrane prevents loss of periplasmic proteins and forms a protective barrier preventing exposure of the bacteria to hydrolytic enzymes and toxic substances such as bile in the gastrointestinal tract. Membrane proteins (porins) are present in the outer membrane and serve to regulate transport through membrane pores. Embedded in the outside layer of the outer membrane protein is lipopolysaccharide (LPS) which is largely responsible for the endotoxic properties of this group of bacteria. Gram-negative cell wall
Endotoxin is synonymous with the Gram-negative cell wall, in particular the LPS component. It is responsible for many of the toxic manifestations of infections with Gram-negative bacilli. The intravascular presence of endotoxin stimulates a cascade of pro-inflammatory cytokine production that produces the dreaded clinical picture of “Gram-negative septicemia” manifest by fever, leukopenia, disseminated intravascular coagulation, shock and often death.
Pili or fimbriae are hair-like projections on the surface of the bacilli that mediate attachment to host cells. For example, only strains of E. coli that possess pili facilitating adherence to urothelial cells cause urinary tract infections.
Escherichia coli are present in the gastrointestinal tract of all people in large numbers and are frequently associated with infections of the urinary tract or the abdominal cavity when gastrointestinal tract integrity has been compromised. Most infections are endogenous. That is, one does not acquire new strains of E. coli and then become ill. The exceptions are toxin mediated diseases associated with particular serotypes (strains) of E. coli discussed below.
The antigenic composition of E. coli is complex, with more than 170 cell membrane (O) antigens, 56 flagellar (H) antigens and numerous capsular (K) antigens described. The serological differentiation of E. coli isolates is not routinely performed but may be useful for epidemiological purposes. The exception is in the active searching for E. coli O:157 H:7 in stool from patients with diarrhea.
E.coli is the cause of a large majority of urinary tract infections and this is the disease that Canadian physicians will most associate with this organism. It is also an important organism in any infective process involving disruption of the GI tract, almost always as part of a polymicrobial (many different bacterial species) process. Examples include appendiceal and diverticular abscesses.
However, there are several types of toxin mediated diseases caused by specific serotypes of E. coli. Most are uncommon in Canada but one particular type has become of great importance to the food industry — enterohemorrhagic E.coli O:157 H:7.
Enterohemorrhagic E. coli
These strains produce a toxin which is responsible for hemorrhagic colitis manifest by bloody diarrhea which is often severe. A small proportion of people with colitis will progress to more serious disease also thought to be toxin mediated. Hemolytic-uremic syndrome (HUS) in children leads to renal failure and is sometimes fatal. A related disease, Thombotic Thrombocytopenic Purpura (TTP), occurs in adults.
This toxin is referred to as verocytoxin or verotoxin because it is detected by its ability to kill cultured vero cells and is produced by several different serotypes of E. coli (sometimes collectively referred to as VTEC — Verocytotoxin producing E. coli). By far, the most important serotype is O:157 H:7. E. coli O:157 H:7 has caused very large food and water borne outbreaks of disease including the massive Walkerton, Ontario municipal water outbreak. Verotoxin producing E. coli are mostly of animal origin, in particular cattle, and the disease has been referred to in the lay press as “hamburger disease”. This is a very important public health issue at present.
E. coli O:157 H:7 is specifically looked for in cultures of diarrheal stool in most laboratories in Canada by culturing on specialized selective and differential media.
Enterotoxigenic E. coli
These strains are common causes of traveler’s diarrhea, particularly in underdeveloped countries. This diarrhea is mediated by two toxins that are similar to the well-characterized Vibrio cholerae toxin with activation of adenylate cyclase and subsequent hypersecretion of fluids into the small intestine. A vaccine has recently become available marketed as Dukoral™. They are genrally not identified in the routine diagnostic laboratory.
Enteroinvasive E. coli
These strains affect the large intestine and can cause dysentery (like Shigella spp.) manifest by blood, mucus and pus in stools. Like enterotoxigenic strains, it is difficult to distinguish these strains from normal commensal E.coli strains and, therefore, they are not commonly recognized.
Further Reading – Todar’s Pathogenic E. coli
Salmonellosis (diarrhea caused by Salmonella spp.) is the prototypic zoonosis — disease in humans caused by contact with organisms of animal origin. Most Salmonella that cause gastrointestinal disease are very closely related genetically and phenotypically. They are grouped together in the species S. enterica and are distinguished from each other serologically into approximately 2200 different serotypes. However, by convention, Genus-serotype names are usually used as Genus-species names for these bacteria. (e.g. Rather than Salmonella enterica – serotype dublin, Salmonella dublin is commonly used)
Salmonella typhi is the Salmonella sp. of human origin and is the cause of enteric fever, which is a very common and serious illness in the developing world and should be considered in travelers returning from endemic areas. It is not a cause of diarrhea and enteric fever should be considered as a completely different clinical entity to Salmonella gastroenteritis.
Salmonella are widely distributed in the animal kingdom. The source of most infections is ingestion of contaminated water or food products, especially poultry. The most common sources of human infections are poultry, eggs and dairy products. Commercially prepared poultry are often contaminated with Salmonella, hence the importance of cooking poultry well. Reptiles kept as pets have become increasingly implicated as sources of infections with unusual serotypes of Salmonella. Outbreaks have also been described secondary to contamination of fresh fruits and vegetables. This is a growing problem as more of our produce is produced in developing countries.
Gastroenteritis is the most common form of Salmonellosis and occurs about 6 – 72 hours after consumption of the contaminated food. Illness is usually relatively short-lived (4-7 days) but severe disease including bacteremia and “metastatic” infection of joints, meninges etc may occur.
Further Reading – CDC Salmonellosis
In the world context, species of the genus Shigella are very common causes of dysentery – diarrhea characterized by significant blood and mucous. However, modern sanitation structures have markedly reduced the incidence of this infection in the developed world. The only reservoir is infected humans and spread is by the fecal oral route either directly or indirectly through contamination of food or water.
Further Reading – CDC Shigellosis
Yersinia pestis is the etiologic agent of plague. People usually get plague from being bitten by a rodent flea that is carrying Y. pestis or by handling an infected animal. Plague is still endemic in parts of the world but is now very rare in North America. During a five year period in the middle of the fourteenth century epidemic plague (the Black Death) claimed 25,000,000 people – almost one quarter of the European population. There are two clinical forms – bubonic and pneumonic. The bubonic plague is characterized by painful, inflamed, swollen lymph nodes in the groin or axillae with fever and bacteremia. The pneumonic form is often lethal, is much more infectious and was responsible for the rapid epidemic spread seen in the squalid conditions of the European middle ages.
Yersinia enterocolitica is a well-recognized but uncommon cause of gastroenteritis. Mesenteric adenitis, a condition that can often be mistaken for acute appendicitis is also a manifestation of infection with this organism.
Further Reading – CDC Plague home page
Serratia, Proteus, Citrobacter and Enterobacter
These four genera are all members of the family Enterobacteriaciae that, collectively, are sometimes referred to as “SPICE” enteric Gram-negative bacilli. These genera are organisms of particular concern in hospitals and are inherently resistant to antibiotics. Selection of antibiotics for hospital-acquired infections is influenced by the types of enteric Gram-negative bacilli that are found in particular institutions.
Vibrio and Campylobacter
The genus Vibrio is comprised of many species of motile, aerobic Gram-negative rods that are found in aquatic environments. Three species are commonly pathogenic for humans, the most important being V. cholerae – the etiologic agent of epidemic cholera. While some cases are mild, the profuse, watery diarrhea of severe cases of cholera can cause death by dehydration in hours. The presence of cholera is a marker of inadequacy of water and sewage treatment and thousands of people still die annually of this preventable illness.
Further Reading – CDC Vibrio cholera
Vibrio parahemolyticus is a marine vibrio that causes gastroenteritis. It is most commonly associated with the consumpion of raw seafood, in particular oysters and is a particular problem along the gulf coast of the United States.
Further Reading – CDC Vibrio parahemolyticus
Vibrio vulnificus is a rare, but often fatal cause of septicemic wound infections associated with sea water particularly in the gulf states of the US. Can also cause gastroenteritis when ingested in seafood similar to V. parahemolyticus.
Further Reading – CDC Vibrio vulnificus
Campylobacter jejuni is a micro-aerophilic (an organism that needs oxygen but at lower partial pressure than atmospheric oxygen) Gram negative bacillus that has been recognized, in the last 20 years, as a very common cause of food-borne gastroenteritis. It was formerly classified as a member of the genus Vibrio. It is now the most commonly identified cause of gastroenteritis in Canada. Poultry is a common source and direct contact with pets, in particular puppies and kittens have been described as sources of transmission.
Further Reading – CDC Campylobacter
Pseudomonas aeruginosa is an ubiquitous organism found in soil, vegetation and water. It is the most important of a large group of environmental organisms commonly referred to as “pseudomonads” that share many phenotypic characteristics. They only utilize oxidative means of respiration and are not able to metabolize substrates via fermentative pathways. Sometimes pseudomonads and similar, but genetically unrelated Gram-negative rods such as Achromobacter spp. and Alcaligenes spp. are referred to as “non-fermenters” in contrast to the Enterobacteriaciae.
P. aeruginosa is the prototypic opportunistic pathogen in that it very rarely causes disease in immunocompetent people but can cause very serious infection of almost any tissue in people with impaired immune function. It is a particularly worrisome organism in patients undergoing cancer chemotherapy and in the Intensive Care Unit. It is inherently resistant to many antibiotics.
Further Reading – Todar’s Pseudomonas aeruginosa