General Overview
Escherichia coli is a gram-negative bacillus which belong to the large group Enterobacteriaceae.
They are found in the human and animal gut as commensals.
These bacteria are indicators of faecal contamination.
Whenever there is any faecal contamination Escherichia coli can be found.
Escherichia coli consists of variety of strains.
E. coli are the most common cause of Urinary tract infections (UTI).
They are commonly isolated from patients who are having cystitis. Infections are most common among women and recurring infections are also can be occurred.
Other infections like endotoxin induced shock, wounds, peritonitis and sepsis are also can be caused by this organism.
Most commonly in neonates, they cause Meningitis and bacteraemia.
Moreover, they can cause diarrhoeal diseases such as infantile gastroenteritis, traveller’s diarrhoea, dysentery, and haemorrhagic diarrhoea.
Haemorrhagic diarrhoea can be progress to haemolytic uraemic syndrome. Â
Escherichia coli consists of variety of strains.
E. coli associated with diarrhoeal disease are Enterotoxigenic E. coli (ETEC), (Enteropathogenic E. coli (EPEC), Enteroinvasive E. coli (EIEC), Enterohaemorrhagic E. coli (EHEC), Enteroaggregative E. coli (EaggEC).
E. coli has several components that contribute for the infection such as pili, capsule, endotoxin and enterotoxins.
Depending on the clinical features and the site of infection Urine, pus, faeces, cerebrospinal fluid (infants), and blood can be used for the diagnosis.
Microscopic Identification
In Gram-stained slides, E. coli can be identified as Gram-negative bacilli. Usually, they are motile.
Culture
coli is an aerobic and facultatively anaerobic bacilli. They optimally grow at 36–37 ºC. However, most strains grow in temperatures between 18–44 ºC.
Blood agar
coli produces mucoid colonies after overnight incubation. The colonies are 1-4 mm in diameter. Some strains of E. coli are haemolytic.
MacConkey agar
Smooth pink colour lactose fermenting colonies can be seen on MacConkey agar. They are 1-4 mm in diameter.
CLED agar
Produce Yellow colour colonies on CLED agar.
Sorbitol MacConkey agar
Most E. coli strains ferment sorbitol.
They produce colurless colonies on sorbitol MacConkey agar.
Some strains are non-sorbitol fermenting. E. coli O157:H7 is sorbitol non fermenting organism.
XLD and DCA agar
On XLD agar yellow colonies are produced. They do not grow on DCA agar.
KIA (Kligler iron agar)
Most strains produce an acid butt and an acid slant (yellow butt and yellow slant). Gas production is normally present.
Usually, hydrogen sulphide (H2S) production is not done by E. coli. Therefore, there cannot be seen any blackening in the medium.
EMB agar
It is a differential medium to differentiate E. coli.
The colonies have a characteristic green color (green sheen).
Biochemical Identification
- Indole positive
- LDC (Lysine decarboxylase) positive (Decarboxylase lysine test)
- Beta-glucuronidase (PGUA) positive ( coli O157:H7 PGUA negative)
- Positive for urine nitrite test
- Citrate Negative
- H2S negative
For the serogrouping of E. coli, it is necessary to have special microbiology laboratories.
When suspecting EHEC O157 in haemorrhagic colitis, a presumptive diagnosis can be made by growing on sorbitol MacConkey agar.
The isolated sorbitol non fermenting E. coli can be confirmed by performing latex agglutination test with O157 antiserum.   Â
Treatment
When there is no any acquired resistance to antibiotics, E. coli can be treated with many antibiotics such as ampicillin, cephalosporins, tetracyclines, quinolones, aminoglycosides, trimethoprim and sulphonamides.
Normally trimethoprim or nitrofurantoin is used for the treatment of uncomplicated cystitis.
However, in serious infections treatment should be based on the laboratory antimicrobial susceptibility test results.
Many strains of E. coli have plasmid mediated acquired antibiotic resistance.
In bacterial meningitis, as it is considered as medical emergency, early treatment should be followed with cefotaxime and gentamicin.
In diarrhoea caused by E. coli, normally treatment with antibiotics is not necessary.
Rehydration of patient should be followed in diarrhoeal disease.
Infections caused by E. coli are treated according to the site of disease and the resistance pattern of the isolated organism.
Prevention of the disease
There are no passive immunization methods to prevent the infection.
When in urinary catheterization and cystoscopy, it should be used aseptic procedures to minimize the introduction of bacteria into the bladder.
Furthermore, prompt removal of urine catheters should be done to minimize urinary tract infections.
However, in recurrent infection it is necessary to give prolonged prophylaxis drugs such as nitrofurantoin or trimethoprim-sulfamethoxazole.
By the prompt removal of intravenous lines, occurring of some sepsis conditions can be prevented.
To prevent the travelers’ diarrhoea prophylactic drugs like doxycycline, ciprofloxacin, trimethoprim-sulfamethoxazole can be used.
Drinking of unpurified water and partially cooked or uncooked foods should be avoided while travelling in risk countries.
No responses yet