Enterobacteriaceae are a homogenous family of aerobic and facultative anaerobic Gram-negative rods found in soil, water, and as part of the enteric bacilli of the large intestine. Of the many different Enterobacteriaceae genera, 12 are important to humans:
- Citrobacter
- Enterobacter
- Escherichia – UTI; enterotoxigenic (children)
- Hafnia
- Klebsiella – UTI; septicaemia; meningitis (neonates)
- Morganella
- Proteus – UTI
- Providencia
- Salmonella – enteric fever; food poisoning
- Serratia
- Shigella – dysentery (neurotoxin)
- Yersinia – plague; septicaemia
Not all Enterobacteriaceae are found in the intestine, however. Many of them are “opportunists”, capable of overcoming non-specific host defenses. Many are associated with nosocomial infection, of which 5-10% of all such cases are caused by one of the Enterobacteriaceae and not uncommonly lead to bacteraemia—bacteria showering the bloodstream from a focus—and, more significantly, septicaemia—bacteria actively multiplying within the bloodstream.
Furthermore, Gram negative sepsis can be rapidly fatal left untreated. Moreover, many Enterobacteriaceae increasingly show resistance to multiple antibiotics. Between 0.5 and 3.0 mcm long, some of these non-spore forming rods are motile, bearing so-called peritrichous flagellae about their entire surface; rather than the single polar flagellum of pseudomonads, for instance.
There are two important non-motile genera of Enterobacteriaceae—Klebsiellae and Shigellae. Klebsiella species are capsulated, enteric organisms (humans and animals) and saprophytes (soil and water) which grow into large mucoid colonies and common causes of nosocomial urinary tract infection, sepsis, and even meningitis. Shigella are intestinal parasites of man and animals that cause serious gastrointestinal infections of bloody diarrhoea. The human pathogen, Shigella flexnerii, for instance, is carried by flies from human faeces. Apart from important virulence factors within their cell wall structure—murein, lipoprotein, phospholipid, protein, lipopolysaccharide—Shigellae have fimbriae and pili which afford their ability to adhere to host-cell surfaces. Lipopolysaccharide (LPS), responsible for the antigenicity of Enterobacteriaceae, is a particularly important virulence factor.
Antigenic Determinants
These are used for diagnostic testing and identifying serotypes within any species:
- “K” antigen: superficial, heat-labile polysaccharide – e.g. K1 subtype of E. coli has a strong association with virulence, especially meningitis in neonates
- “O” (German: ohne; “without”) antigen: lipopolysaccharide (endotoxin), a thymus-independent antigen stimulating the production of IgM
- “H” (German: Hauch; “breath” or “mist”) antigen: a flagellae protein, also thymus-independent antigen, which stimulates the production of IgG
Agglutination of organism by way of “O” and “H” antigens detects the presence of IgM or IgG antibody (Widal Test) in the patient’s serum.
Pathology of Infection
Gram negative cell wall
- Lipoprotein
- LPS: major component of outer membrane (image above); now known to also be “shed” as vesicles (image below)
- Peptidoglycan molecules
“O” antigen, the chief surface antigen, can be used as a surrogate marker of the clinically significant virulence factor, LPS, responsible for the high mortality of Gram-negative infection.
- Surface “O” Antigen: polymer of oligosaccharides
- Core oligosaccharide: endows LPS its antigenicity and unique for “O” antigen
- Lipid A moiety: responsible for virulence / toxic properties of endotoxin
Most of the pathophysiology of Gram-negative infection then relates to endotoxin, which is considered the prime cause of:
- Fever
- Induction of fatal shock – i.e. disseminated intravascular coagulation (DIC)
- Leucocyte alterations – endotoxins are extremely powerful stimulators of the reticuloendothelial system
- Regression of established tumours – tumour necrosis factor (TNF)*
- Accelerate growth of tumours – i.e. promotes angiogenesis
- Haemorrhagic tissue necrosis
- Alter responses to infections – e.g. vaccine used for immunisation against typhoid fever (TAB) is the killed Salmonella typhi microorganism loaded with endotoxins which means it can potentially be of use within the midst of endemic infection
- There is some suggestion that these vaccines can activate extant latent host infections from other microorganism, such as tuberculosis, by somehow upsetting the host-microorganism immune status quo
* TNF encourages central tumour necrosis by occluding blood vessels with platelets
According to the current model, the specific cellular recognition of agonistic LPS/lipid A is initialized by the combined extracellular actions of LPS binding protein (LBP), the membrane-bound or soluble forms of CD14 and the newly identified Toll-like receptor 4 (TLR4)*MD-2 complex, leading to the rapid activation of an intracellular signaling network that is highly homologous to the signaling systems of IL-1 and IL-18.¹
Physiological and pharmacological effects of endotoxins:
| TXA2 | PGI2 | PGs | 5, 12-Di-HETE (LTB4) | LTC4 à LTD4 |
| thromboxane | prostacyclin | E2, F2, D2 | powerful chemotaxin | SRS-A |
| Thrombosis | Thrombolysis | Smooth muscle contraction | Neutrophil/eosinophil chemotaxis | Smooth muscle contraction |
| Platelet aggregation | pain | C3b Receptor expression | Pulmonary/tracheal constriction | |
| vasoconstriction | vasodilatation | vasodilatation | Lysosomal enzyme release | Increase microvascular permeability |
- LTB4: 5,12-Dihydroxy-icosatetraenoic acid
- LTD4: slow-reacting substance of anaphylaxis (SRS-A)
Endotoxins initially act to dampen the host immune response
Endotoxins initially (within hours) reduce the activity of granulocytes, by interfering with diapedesis, allowing organisms in the extracellular space to escape, but hopefully later (days) activation of certain cells of the immune system, such as macrophages (mφ), catches up with them. That is, on initial exposure, there is a temporary depression of non-specific host defenses.
| Hours | 1 week |
| Reduced granulocyte function | Increased granulocyte function |
| Reduced microorganism clearance | Increased microorganism clearance |
| Reduced resistance | Increased non-specific resistance (mφ) |
| Reduced complement (c) |
Endotoxin (LPS) activates complement via the alternate pathway:
Complexes of complement and specific Ab → lysis of Gram-negative organisms (and red blood cells).
Corticosteroids probably act much lower down the pathway; it is now thought. Aspirin and other NSAIDs not only do not affect LT/PAF pathway but they actually induce—through their effect on TXA2 and the prostaglandin pathway—leukotriene production.
IL1, like C-reactive protein (CRP), is an acute phase reactant (APR); that is, both are antibody-like substances. While it may be desirable to block the effects of PAF, it is not desirable to suppress acute-phase reactants. In which case corticosteroids may have a place yet they may also be contraindicated, in septicaemia, because of their effect to reduce leucocyte (granulocyte) function.
Risk factors associated with mortality from septic shock include, for instance, cirrhosis, because the liver has an important role in endotoxin detoxification. Impairment of liver function, such as in those with a long history of significant alcohol consumption, afford endotoxin spill-over with resultant deleterious systemic effects. Such a patient who undergoes an elective laparoscopic cholecystectomy, for instance, is not only high-risk for Klebsiella pneumonitis (or infection with another capsulated organism, such as Serratia marcescens) but also represents a poorer prognosis from such a complication than an otherwise healthy person. Essentially, LPS (endotoxin) cripples host clearance mechanisms. If such premorbid mechanisms are already functioning poorly, that person will rapidly go into shock should infection complicate recovery.
References
Alexander, C. and Rietschel, E. “Bacterial lipopolysaccharides and innate immunity.” Journal of endotoxin research 73 (2001): 167-202. Available at https://journals.sagepub.com/doi/pdf/10.1177/09680519010070030101.
Hsu, Yen-Ming. “Regarding Endotoxin.” Science Blog. AB Biosciences. June 23, 20202. Available at https://abbiosciences.com/blogs/news/regarding-endotoxin.
Orskov Frits; Orskov, Ida. Chapter I Serotyping of Enterobacteriaceae, with Special Emphasis on K Antigen Determination. Methods in Microbiology 11, 1978: 1-77. Available at https://doi.org/10.1016/S0580-9517(08)70486-0.
Kearney, R. “Enterobacteriaceae.” Lecture, University of Sydney, Sydney, July 26, 1990.
Appendix – Eicosanoid synthesis
[Image: Wikimedia Commons]
Further Reading
Radia T. Jamil; Lisa A. Foris; Jessica Snowden. Proteus Mirabilis Infections. StatPearls [Internet]. Available at https://www.ncbi.nlm.nih.gov/books/NBK442017/.
Todar, Kenneth. Pathogenic E. coli . Available at www.textbookofbacteriology.net.
