Phylum Proteobacteria - Order Enterobacteriales
Here we discuss characteristics & importance of the main generas of this Enterobacteriales order such as Salmonella, Escherichia and etc.
General characteristics of Enterobacteriales
- Cell Shape – Rod Shaped
- Cell wall type – Gram negative
- Respiratory pattern – Facultative anaerobic
- Endospore production – Non-spore forming
- Motility – Some species only motile.
- Oxidase Enzyme – Present
- Capsule formation – Some produce capsules : Ex- Klebsiella
- Habitat – Lives in soil, Water, and in plant and animal bodies.Vastly distributed Order.
- Needs special growth factors – Some of these organisms need specific growth factors (especially vitamins) to grow. This is because they cannot synthesize their vitamins.
Sugar Fermentation Pattern of Enterobacteriales
Members of the order Enterobacteriales show two different pathways of sugar fermentation;
- Mixed acid fermentation pathway
- Butanediol fermentation pathway
Mixed acid fermentation pathway
This is an anaerobic fermentation where the end products are a mixture of acids, (particularly Lactic, Acetic, Succinic & Formic acid) and Ethanol. Some bacteria that perform mixed acid fermentation produce a special enzyme called Formic Hydrogen Lyase which can Lysis formic acid to CO2 and H2 which end up showing a production of gases. Mixed acid fermentation is characteristic for family Enterobacteriacea.
Mixed acid + Gas : Genera Escherichia, Salmonella, Proteus
Mixed acid + no Gas : Genera Yersinia and Shigella
Butanediol fermentation pathway
In this fermentation pathway, form mixed acid and gases as mixed acid fementation pathway, but in addition 2,3- butanediol from the condensation of 2-pyruvate. The benifit of this pathway decreases the acid formation (Butanediol is a neutral product) and causes the formation of a distinctive intermediates (Acetoin). There are specific tests to detect low acid and Acetoin in order to distinguish non-fecal enteric bacteria (Butanediol formers) such as Klebsiella and Enterobacter from fecal enterics (Mixed acid fermenters) such as Escherichia, Salmonella and Shigella.
Butanediol forming bacteria: Genera Serretia, Erwinia, Enterobacter & Klebsiella
Some pathogenic species of Order Enterobacteriales use Type III Secretion system as their mechanism of causing disease.
Type III Secretion System (T3SS/Injectosome/Injectisome)
In several gram negative bacteria have a Type III secretion system (like protein appendage). In pathogenic bacteria, this needle-like secretion structure is used as a sensory probe to detect the presence of eukaryotic organisms and secrete proteins that help the bacteria to infect them. Those proteins (Ex.toxins) are secreted directly from the bacterial cell to the host cytoplasm, where they exert a number of effects that help the pathogen to survive and escape an immune response.
In T3SS, toxins are directly injected to the host cytoplasm through the needle-like structure. Therefore small amount of toxins are enough to cause a fetal disease.
A schematic of the type III secretion-system needle-complex.
Special characteristics of the economically important genera of the Enterobacteriales
- Gram negative rods. Common inhabitants of intestinal tract of many animals, especially poultry and cattle.
- Under unsanitary conditions, these organisms can contaminate food.
- Almost all members of the genus Salmonella are potentially pathogenic.
Accordingly, extensive biochemical and serological tests have been developed to clinically isolate and identify Salmonellae.
The nomenclature of the genus Salmonella is unusual. Instead of multiple species, members of this genus that are infectious to warm-blooded animals can be considered for practical purposes to be a single species, Salmonella enterica. This species is divided in to more than 2400 serovars (Serological varieties). Therefore, a serovar such as Salmonella typhimurium is not a species and should be more properly written as Salmonella enterica Serovar Typhimurium.
Salmonella typhi – Causative agent of Typhoid fever
Salmonella typhimurium – Causative agent of Salmonellosis
Salmonella bangori – Animal pathogen
- Species of the genus Shigella are responsible for a disease called bacillary dysentery or Shigellosis in human, thus they are human pathogens.
- Some species of Genus Shigella can cause life-threatening dysentery.
- Gram negative rods.
- Common inhabitants of Human intestinal tract.
- They are not usually pathogenic. But can be opportunistic pathogenic. However these bacteria can cause different diseases such as Urinary tract infections and Diarrhea – certain strains can produce Enterotoxins that cause diarrhea, and occasionally cause very serious food- borne disease.
Entero-pathogenic Escherichia coli – Infantile diarrhea
Entero-hemorra Escherichia coli – Hemolytic diarrhea
Entero-toxigenic Escherichia coli – Traveller’s diarrhea
Entero-invasive Escherichia coli – Shigella-like diarrhea
- Gram negative rods, Facultative anaerobic.
- They are usually non-pathogenic. But some strains can be pathogenic and cause opportunistic infections in immune – compromised hosts (usually hospitalized) and those who are on mechanical ventilation. The urinary and respiratory tracts are the most common sites of infections.
- The genus Enterobacter is a member of the coliform group of bacteria. But unlike Escherichia, they belong to non-fecal coliforms. (Non-fecal coliforms are incapable of growing at 44.5 °C in the presence of bile salts)
Ex: Enterobacter aerogenes, Enterobacter cloacae
IMViC test method is used to differentiate between E.coli and Enterobacter spp.
|Genus||Indole Test||MR Test||VP TEST||CITRATE TEST|
- Gram negative Rod shaped bacteria
- Plant pathogens.
Erwinia cartivora, Erwinia amylovora – causative agent of dry necrosis on apple. Pears and other Rosaceae crops.
- Gram negative, Rod shaped bacteria
- Facultative anaerobes.
- They are pathogenic species. Some are pathogenic to humans where as others are pathogenic to various other mammals. Rodents are the natural reservoirs of Yersinia spp. Where occasionally other mammals also serve as hosts.
- Yersinia infection may occur either through blood (in case of Yersinia pestis) or in alimentary fashion occasionally through consumption of food products (especially vegetables, milk derived products and meat) contaminated with infected urine or feces.
Yersinia pestis – Causative agent of Plague
Yersinia enterocolitica – Causative agent of Yersiniosis
- Yersinia is implicated as one of the causes of Reactive arthritis and also the genus is associated with Pseudo appendicitis, which is an incorrect diagnosis of Appendicitis due to similar presentation.
- An interesting feature peculiar to some Yersinia spp. Is that their ability to not only to survive but also to actively proliferate at temperatures as low as 1-4 °C.
- These bacteria are quickly inactivated by oxidizing agents such as H2O2 and KMnO4.
Economic importance of Order Enterobacteriales
Species of order Enterobacteriales are economically significant in several ways, from symbionts of the intestinal tract to the most common pathogens cause gastro-intestinal diseases and to the most common food and water spoilage organisms and pathogens to economically important crops.
1.Water quality testing
In testing fecal pollution in natural water resources, water microbiologists are using Escherichia coli as an indicator organism. Although Escherichia spp. are not normally pathogenic, this is done because the test methods which test for most of the harmful enteric bacteria such as Salmonella, and Shigella etc. are time consuming and therefore they are inapplicable where rapid identification is needed.
Some genera of family Enterobacteriaceae are common food-borne pathogens and some of them can cause serious gastro-intestinal diseases that could occasionally be fatal. Genera Salmonella, Shigella are the common food-borne pathogens in this family and although normally non-pathogenic, Escherichia spp. can also cause opportunistic infections in the gastro- intestinal tract and less frequently some urinary tract infections.
Salmonella typhi – Typhoid fever (Yellow fever)
Salmonella typhimurium – Salmonellosis
Escherichia coli – Watery Diarrhea
Some species of, especially genus Erwinia are common pathogens to some economically important plants such as carrot, apple, pears etc.
Gerard J.Tortora, Berdell R.Funke, C. L. C. (2010). MICROBIOLOGY AN INTRODUCTION (10th ed.). Pearson Education,Inc.
Michael T. Madigan, John M. Martinko, Kelly S. Bender, Daniel H. Buckley, D. A. S. (2015). BROCK BIOLOGY OF MICROORGANISMS (14th ed.). Pearson Education,Inc
Pasindu Chamikara – Microbiologist