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Coliform Bacteria – Definition, Classification, Examples

What are Coliforms? 

  • Coliform bacteria are either motile or non-motile Gram-negative, non-spore-forming Bacilli with -galactosidase that create acids and gases at their optimal growth temperature of 35-37°C.
  • Aerobes and facultative aerobes are a common sign of poor hygienic quality in food, milk, and water.
  • Coliforms are present in the aquatic environment, soil, and vegetation; they are universally abundant in the faeces of warm-blooded animals since they are known to occupy the digestive tract.
  • Although coliform bacteria do not typically cause severe sickness, they are simple to cultivate, and their presence indicates that other pathogenic organisms of faecal origin may be present in a sample, or that the material is unsafe for consumption.
  • These pathogens include bacteria, viruses, protozoa, and several multicellular parasites that cause disease.


  • Enterobacteriaceae species are ubiquitous bacteria.
  • They are present in the soil, water, and plants across the globe.
  • Humans and animals also have them as part of their regular intestinal flora. Members of this family are Gram-negative, nonsporing, nonacid-fast, and modestly sized bacilli.
  • They are either motile by means of peritrichous flagella or nonmotile without flagella.
  • They are aerobic and facultatively anaerobic, grow rapidly on standard media, ferment carbohydrates with the generation of acid and gas or acid exclusively, reduce nitrate to nitrite, are catalase-positive but oxidase-negative, and are catalase-positive but oxidase-negative.
  • The oxidase test is an essential test for distinguishing Enterobacteriaceae from numerous other fermentative and nonfermentative Gram-negative bacilli.
  • Members of the family exhibit a great deal of biochemical and antigenic diversity. Enterobacteriaceae microbes are responsible for a variety of human diseases:
    • Certain members of the family, such as Salmonella and Shigella, invariably cause disease in humans.
    • Other species, such as Escherichia coli, Klebsiella pneumoniae, Proteus mirabilis, etc., that are part of the typical human gut flora can also cause infections in other parts of the human body.
    • In addition to this, there is a second category of Enterobacteriaceae organisms that are normal human commensals but turn pathogenic when they acquire virulence factor genes via plasmids, bacteriophages, or pathogenicity islands. E. coli connected with human gastroenteritis is an example.
  • Enterobacteriaceae organisms can be obtained via humans (Salmonella Typhi, Shigella species, etc.), animals (Salmonella species and Yersinia species), or endogenous infection.
  • In the latter case, the pathogen (E. coli) can spread from the initial site of infection to almost all body sites.
Schematic diagram showing variety of diseases caused by the members of the family Enterobacteriaceae in humans.
Schematic diagram showing variety of diseases caused by the members of the family Enterobacteriaceae in humans.

Classification of Coliforms

  • Historically, the characteristics of colonies on regularly used media (such as MacConkey medium) were used to identify and categorise members of the family Enterobacteriaceae.
  • Depending on their ability to ferment lactose, the colonies were classified as lactose-fermenting bacteria (Escherichia spp., Klebsiella spp., Enterobacter spp., Citrobacter spp., etc.) or nonlactose-fermenting bacteria (Salmonella spp., Shigella spp., Proteus spp., etc.).
  • This was utilised in a diagnostic laboratory as a practical procedure.
  • Currently, however, bacteria are classified based on a variety of morphological, biochemical, serological, and DNA-based criteria.
  • Bergey’s manual, Kauffmann, and Edwards–classifications Ewing’s are the three most popular methods for classifying family Enterobacteriaceae members.
  • Approaches to the classification of bacteria are largely identical across all of these methods.
  • According to these techniques, the Enterobacteriaceae family is classified into numerous large groupings or tribes. Each tribe is comprised of multiple genera or subgenera.
  • Each genus has numerous species, which are categorised into various categories, including biotypes, serotypes, colicin types, bacteriophage types, etc.
  • According to the current classification (Ewing 1986), the family Enterobacteriaceae is divided into the eight tribes shown in Table.
  • In the family Enterobacteriaceae, the tribe Yersinieae contains the species Yersinia pestis, which is the causal agent of a significant disease plague.
Ewing’s classification of the family Enterobacteriaceae
Ewing’s classification of the family Enterobacteriaceae
  • In Table, differentiating characteristics of distinct genera of the family Enterobacteraceae are listed.
Important properties distinguishing members of the family Enterobacteriaceae
Important properties distinguishing members of the family Enterobacteriaceae
  • Infections in humans caused by common members of the Enterobacteriaceae family are outlined in Table.
Human infections caused by common members of the family Enterobacteriaceae
Human infections caused by common members of the family Enterobacteriaceae


  • Escherichia are intestinal pathogens in both animals and humans.
  • Five species make up the genus Escherichia: E. coli, Escherichia fergusonii, Escherichia hermanii, Escherichia vulneris, and Escherichia blattae.
  • E. coli is the most prevalent and significant species responsible for human infections.
  • Based on O, H, and K antigens, E. coli is further classified into biotypes and serotypes.


  • Edwardsiella is distinguished from Escherichia by its capacity to create hydrogen sulphide.
  • The sole harmful species for humans is Edwardsiella tarda, which belongs to the genus Edwardsiella.
  • E. tarda resides in the gastrointestinal tracts of snakes and other cold-blooded animals.
  • The word tarda relates to the bacteria’s slow or weak fermentation of carbohydrates.
  • E. tarda is a Gram-negative, noncapsulated, motile, and fermentatively ineffective bacillus.
  • It ferments only glucose and maltose, producing acid and a small amount of gas. It is positive for indole, H2 S, and citrate and decarboxylates lysine and ornithine.
  • E. tarda is a rare human pathogen identified from fatal cases of meningitis from wounds, blood, and CSF.
  • The bacteria have also been isolated from the faeces of healthy individuals and those with diarrhoea.
  • However, the pathogenic role of the bacteria in causing diarrhoea has not yet been determined.


  • Citrobacter is a common occupant of the human intestine.
  • Citrobacter is a genus that contains three species: Citrobacter freundii, Citrobacter amalonaticus, and Citrobacter koseri (formerly C. diversus).
  • They thrive on nutrient agar and other common media, creating colonies that are smooth and convex. The colonies lack pigmentation.
  • On MacConkey and DCA medium, pale colonies are produced.
  • Citrobacter spp. are motile, H2 S positive, MR positive, citrate positive, and have varying indole content.
  • The vast majority of strains decarboxylate ornithine, but not lysine.
  • They digest lactose very slowly or not at all.
  • Due to their great antigenic similarity to salmonellae, they may be misidentified as salmonellae.
  • Vi antigen is closely related to the antigens of Salmonella Typhi and Salmonella Paratyphi.
  • Citrobacter spp. are capable of causing infections in the urinary tract, gallbladder, middle ear, and meninges. C. koseri can occasionally cause neonatal meningitis.
Important properties used for differentiation of Citrobacter species
Important properties used for differentiation of Citrobacter species


  • Klebsiella is a member of the Klebsielleae tribe under the Enterobacteriaceae family.
  • The bacteria are named after Edwin Klebs, who was the first to demonstrate them.
  • Gram-negative, rod-shaped, nonmotile bacteria with a conspicuous polysaccharide capsule are members of the genus Klebsiella.
  • Various changes have been made to the classification of Klebsiella.
  • Previously, the genus Klebsiella was divided into three major species based on metabolic reactions.
  • Currently, they have been categorised into seven species based on DNA homology, namely (a) Klebsiella pneumoniae, (b) Klebsiella ozaenae, (c) Klebsiella rhinoscleromatis, (d) Klebsiella oxytoca, (e) Klebsiella planticola, (f) Klebsiella terrigena, and (g) Klebsiella ornithinolytic
  • Klebsielleae species have emerged as significant agents of nosocomial infections in recent years.
  • K. pneumoniae is the most significant species in the group that causes human illnesses. Infrequently, K. oxytoca and K. rhinoscleromatis have also been linked to human infections.
Important properties used for differentiation of Klebsiella species
Important properties used for differentiation of Klebsiella species

Klebsiella rhinoscleromatis

  • Rhinoscleroma produced by K. rhinoscleromatis is a nasopharyngeal chronic inflammatory illness. Infections with K. rhinoscleromatis are typically observed in Southeast Europe, Central America, and India, but have a global range.
  • Patients may appear with purulent nasal discharge accompanied by the formation of crusts and nodules, which may result in respiratory obstruction.
  • Bacilli are observed intracellularly in lesions, which are isolable and identifiable through biochemical processes.
  • The diagnosis is based on a positive blood culture and histology. The antibiotic rifampin has been used to treat rhinoscleroma.

Klebsiella ozaenae 

  • Chronic atrophic rhinitis caused by K. ozaenae and characterised by necrosis of nasal mucosa and mucopurulent nasal discharge.
  • It commonly affects older individuals. Nasal congestion and a persistent nasal odour are typical symptoms.
  • Unlike rhinoscleroma, however, nasal congestion is not a prevalent symptom.
  • Patients may also have headaches and other sinusitis-related symptoms.
  • It is difficult to identify K. ozaenae because biochemical reactions of isolated strains vary widely.
  • The ozena infection is treated with trimethoprim and sulfamethoxazole.

Klebsiella oxytoca 

  • K. oxytoca can infrequently be isolated from clinical samples.
  • It is increasingly separated from neonatal septicemia patients.
  • Additionally, the bacteria have been linked to neonatal bacteremia, particularly in premature infants and neonatal intensive care units.


  • Twelve species comprise the genus Enterobacter, with Enterobacter cloacae and Enterobacter aerogenes, followed by Enterobacter sakazakii, being the most commonly isolated species causing human infections.
  • Additionally, Enterobacter asburiae, Enterobacter gergoviae, Enterobacter taylorae, and Enterobacter hormaechei are occasionally linked to human illnesses. E. cloacae and E. aerogenes are two of the most significant Enterobacter species that cause a range of nosocomial illnesses.
  • Enterobacter are Gram-negative, Klebsielleae-belonging bacilli that are both aerobic and facultatively anaerobic.
  • On sheep blood agar, Enterobacter generates big, grey, dry, or mucoid colonies; on MacConkey agar, it produces pink colonies with lactose fermentation. The bacteria produce acid by fermenting glucose.
  • They vary from Klebsiella in that they are motile, negative for urease, and positive for ornithine decarboxylase.
  • Endotoxin of the bacterium is recognised to play a significant role in the aetiology and consequences of sepsis.
  • Enterobacter species rarely infect otherwise healthy individuals. These pathogens are opportunistic.
  • Long-term hospital patients, particularly those in the ICU, have an elevated risk of developing an Enterobacter infection.
  • Patients with serious underlying illnesses (e.g., diabetes, cancers, burns, mechanical ventilation), foreign devices (e.g., intravenous catheters), and immunosuppression are also at an elevated risk of bacterial infection.
  • They induce frequent and severe nosocomial infections, including urinary tract infections, lower respiratory tract infections, skin and soft tissue infections, bacteremia, endocarditis, intraabdominal infections, septic arthritis, and osteomyelitis, in these individuals.
  • These infections are linked to prolonged hospitalisation, a range of surgical and nonsurgical procedures, and the use of new and costly antimicrobial medicines.
  • These bacteria cause major morbidity and death, and their numerous drug resistances hamper infection therapy.
  • These bacteria possess inducible beta-lactamases, which are undetectable in vitro but cause resistance during treatment.
  • There are both endogenous and external causes of infections.
    • The aetiology of endogenous Enterobacter infections is a bacterial colonisation of the skin, gastrointestinal system, or urinary tract.
    • The hands of medical workers, intravenous solutions, endoscopes, blood products, instruments for monitoring intraarterial pressure, and stethoscopes are common sources of Enterobacter-caused external infections.
  • The diagnosis is determined by repeated culturing of suitable clinical specimens. Blood culture is beneficial for isolating germs from people with bacteremia.
  • The most often used antibiotics against Enterobacter infections are carbapenems, cephalosporins of the fourth generation, aminoglycosides, novel quinolones, and trimethoprim–sulfamethoxazole (TMP–SMX).
  • Cephalosporins of the third generation often demonstrate good in vitro effectiveness against these species, but are associated with an increased risk of developing complete resistance during therapy.
  • Carbapenems are the most effective antibiotics against E. cloacae, E. aerogenes, and other bacteria.
  • Infections caused by Enterobacter are not treated with first-generation or second-generation cephalosporins.
Differentiation of Enterobacter species
Differentiation of Enterobacter species


  • The single species of the genus Hafnia is Hafnia alvei.
  • It is present in the faeces of humans and animals, sewage, soil, and water. H. alvei is motile.
  • It does not ferment lactose, raffinose, sucrose, adonitol dulcitol, and inositol.
  • It is negative for indole and MR and positive for VP and citrate.
  • At 22°C, biochemical reactions are easier to observe than at 37°C.
  • Bacteria have been identified from abscesses, wounds, sputum, urine, and blood, among other places, but frequently in association with other bacteria.
  • H. alvei’s pathogenic role has yet to be determined.


  • Serratia are Gram-negative bacteria belonging to the Klebsielleae tribe. The only pathogenic species that causes human infection is Serratia marcescens.
  • In 1819, Bartolomeo Bizio, an Italian pharmacist from Padua, was the first to identify S. marcescens as the agent responsible for the crimson colouring of polenta.
  • The bacteria was named Serratia after the Italian scientist who invented the steamboat, Serrati.
  • The species name marcescens is derived from the Latin word for decaying because the bloody pigment produced by the bacteria degrades rapidly.
  • Since the 1960s, S. marcescens has been recognised as a human-infecting opportunistic pathogen.
  • Depending on the age of the colonies, several strains of S. marcescens produce a pigment termed prodigiosin that ranges in hue from dark red to pink or magenta.
  • Typically, S. marcescens grows on starchy foods, where the development of pigmented colonies can be mistaken for blood droplets.
  • S. marcescens is pleomorphic, exhibiting both coccobacillary and bacillary forms. Serratia typically colonises the respiratory and urinary systems of adult hospital patients.
  • Nearly 2% of nosocomial infections of these patients’ urinary system, lower respiratory tract, surgical wounds, blood, skin, and soft tissues are caused by this bacteria.
  • Meningitis, wound infections, and arthritis epidemics have been linked to S. marcescens in paediatric wards and intensive care units.
  • The bacteria also causes endocarditis and osteomyelitis in intravenous drug addicts, such as heroin addicts.
  • Serratia infections are more likely to be severe in the elderly, those with a history of antibiotic use, and those with chronic or debilitating conditions. Amikacin and quinolones are effective against S. marcescens, while gentamicin and tobramycin are ineffective.
  • The bacteria are resistant to ampicillin, macrolides, and cephalosporins of the first generation.
  • Therefore, the treatment of S. marcescens is determined by antibiotic susceptibility testing findings.


  • Along with Morganella and Providencia, the genus Proteus is a member of the tribe Proteeae.
  • The term “Proteus” relates to their pleomorphic feature, which is named after the Greek god Proteus, who could assume any form.
  • With few exceptions, all members of the tribe Proteeae are Gram-negative, noncapsulated, pleomorphic, and motile bacilli. With the exception of certain Providencia strains, the majority of these bacteria produce the enzyme urease, which hydrolyzes urea into ammonia and carbon dioxide.
  • They breakdown tyrosine, are MR positive and VP negative, and proliferate in the presence of KCN.
  • They do not decarboxylate amino acids like arginine, lysine, or ornithine dehydrogenase.
  • They neither ferment lactose nor dulcitol, nor use malonate. The production of the enzyme phenyl alanine deaminase, which converts phenyl alanine to phenyl pyruvic acid (PPA reaction), distinguishes Proteeae from other members of the Enterobacteriaceae family.
  • There are four species within the genus Proteus: Proteus mirabilis, Proteus vulgaris, Proteus penneri, and Proteus myxofaciens. P. mirabilis is the most significant species, causing 90% of Proteus infections and being linked to urinary tract and wound infections acquired in the community.
  • Typically, P. vulgaris and P. penneri are linked to hospital-acquired illnesses. They are separated from patients with chronic debilitating conditions and immunocompromised individuals.
Differentiation of genera of the Tribe Proteeae
Differentiation of genera of the Tribe Proteeae


  • The genus Morganella is a member of the Proteeae tribe.
  • The only species in the genus Morganella is Morganella morganii, which has two subspecies: morganii and sibonii. Previously, M. morganii was assigned to the genus Proteus as Proteus morganii. M. morganii are small, Gram-negative, motile bacilli; nevertheless, unlike Proteus species, they do not cause swarming on solid medium.
  • They possess facultative anaerobiosis and lack cell walls.
  • On blood agar or MacConkey agar, they proliferate.
  • They are negative for oxidase and positive for catalase and indole. glucose and mannose are fermented by M. morganii, but not lactose.
  • The bacteria decrease nitrates through decarboxylating ornithine, hydrolyzing urease, and hydrolyzing urease.
  • They are incapable of liquifying gelatin and producing hydrogen sulphide. M. morganii is frequently found in human and animal faeces and causes serious invasive illnesses seldom.
  • It is most frequently identified as an opportunistic pathogen in hospitalised patients, especially those receiving prolonged antibiotic therapy. M. morganii causes urinary tract infections, which are frequently linked to an alkaline urine pH.
  • Infrequently, the bacteria have been linked to sepsis, pneumonia, wound infections, pericarditis, chorioamnionitis, endophthalmitis, empyema, spontaneous bacterial peritonitis, and central nervous system infections.
  • Nosocomial M. morganii strains are frequently sensitive to cefepime, imipenem, meropenem, piperacillin, aminoglycosides, and fluoroquinolones.
  • These have also demonstrated resistance to ceftazidime and other cephalosporins of the third generation. M. morganii strains that produce ESBL have been reported recently.


  • The genus Providencia consists of five species: Providencia stuartii, Providencia rettgeri, Providencia alcalifaciens, Providencia rustigianii, and Providencia heimbachae.
  • On solid media, Providencia spp. are Gram-negative, motile bacilli that do not swarm.
  • They emit a pleasant odour and create yellow to orange colonies on DCA. All species deaminate phenylalanine, but only P. rettgeri regularly hydrolyzes urea.
  • Table 31-11 summarises Providencia’s further biochemical features. Human urine, faeces, and blood, as well as the neck, perineum, axilla, and wounds, have all yielded Providencia species.
  • P. stuartii is the most prevalent species responsible for human infections. P. stuartii and, to a lesser extent, P. rettgeri are regularly discovered in individuals having indwelling urinary catheters for an extended period of time.
  • P. stuartii and P. rettgeri are connected with the use of urinary catheters, which is significantly more prevalent among the elderly. As a result, older individuals are at a greater risk of infection.
  • Nearly sixty percent of the bacterial pathogens recovered from the urine of these individuals are P. stuartii. P. stuartii contains adhesin, mannose-resistant/Klebsiella-like (MR/K) hemagglutinin protein, enabling it to adhere to the urine catheter.
  • P. stuartii can spread from urine to blood, creating a frequent infection of the bloodstream in elderly and immunocompromised people. P. alcalifaciens, P. rettgeri, and P. stuartii also may cause invasive diarrhoea.
  • These organisms are gaining prominence as significant causes of traveler’s diarrhoea in adults. Urine and faeces culture are routinely used to diagnose a UTI and diarrhoea.
  • Blood culture is important for diagnosing suspected infections of the bloodstream. Due to the fact that numerous Providencia species are resistant to multiple medications, antibiotic susceptibility testing is useful for determining which drugs should be used for therapy. P. stuartii is the Providencia species with the highest resistance.
  • It demonstrates resistance to tetracyclines, older penicillins, cephalosporins, fluoroquinolones, aminoglycosides, and TMP–SMX.
  • It is vulnerable to cephalosporins of the latest generation, aztreonam, and carbapenems. P. alcalifaciens and P. rustigianii are typically antibiotic-susceptible.
  • They are typically sensitive to fluoroquinolones, aminoglycosides, late-generation cephalosporins, aztreonam, carbapenems, and TMP–SMX.
  • They are resistant to tetracyclines, penicillins of the past, and cephalosporins.


  • Erwinia organisms are typically found in soil and cause plant infections.
  • Erwinia herbicola is the sole species that has been occasionally isolated from respiratory and urinary infections in chronically ill hospitalised patients and patients with chronic debilitation.

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Why do Laboratory incubators need CO2? What is Karyotyping? What are the scope of Microbiology? What is DNA Library? What is Simple Staining? What is Negative Staining? What is Western Blot? What are Transgenic Plants? Breakthrough Discovery: Crystal Cells in Fruit Flies Key to Oxygen Transport What is Northern Blotting?
Why do Laboratory incubators need CO2? What is Karyotyping? What are the scope of Microbiology? What is DNA Library? What is Simple Staining? What is Negative Staining? What is Western Blot? What are Transgenic Plants? Breakthrough Discovery: Crystal Cells in Fruit Flies Key to Oxygen Transport What is Northern Blotting?
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