Escherichia coli is a facultative, enteric, Gram-negative, motile/flagellated, and lactose-fermenting rod that occur in the genus Escherichia and family Enterobacteria or Enterobacteriaceae. Enterobacteriaceae are bacteria that naturally exist in the intestinal tract of animals and humans, and also found in water and soil. Because the natural habitat of E. coli is the intestinal tract of humans (animals inclusive), it is therefore used as an indicator of the feacal contamination of drinking water and water used for other domestic and industrial purposes. As a baseline, 100 ml of drinking water must not contain any trace of E. coli. While most enteric Gram-negative bacteria such as Shigella and Salmonella are important and regular human pathogens, E. coli are members of the normal intestinal flora and may only cause disease by chance.
Illustration of the Gram negative staining of E. coli. Under the microscope, E. coli appears as a rod-shaped organism with a pink or red colour.
PATHOGENESIS OF ESCHERICHIA COLI INFECTION
E. coli is the most common normal flora of the intestinal tract of humans, and they can also be found in the genital tract and upper respiratory tracts in traces. The presence of pathogenicity islands (acquired foreign DNA) in the genome of enteropathogenic E. coli enhances the pathogenicity and/or virulence of the organism. Infections resulting from E. coli occur occasionally and this has been attributable to the relocation of the bacteria from its normal location in the intestine to other extra-intestinal sites. E. coli causes UTI, diarrheal diseases, meningitis, wound infections and peritonitis. They produce various types of toxins including shiga toxin, labile toxin, stable toxin amongst others; and these help to increase their virulence in the host cells.
Though E. coli is implicated in a handful of human infections when they have the opportunity, diarrheal diseases and UTIs are amongst the two most important infections that characterize the majority of hospital visits across the world. Urinary tract infections caused by E. coli usually arise from the entry of uropathogens into the bladder. Uropathogens can gain entry into the human bladder through sexual intercourse or some minor strain experienced during sexual activity. Urinary tract infections (UTIs) are caused by the presence and growth of bacteria anywhere in the urinary tract system including the kidney, bladder and the urethra. Most cases of UTIs are asymptomatic but some are symptomatic and may present with some clinical signs and symptoms such as increased frequency of urination, dysuria and haematuria (i.e. blood in urine). UTIs affect either the upper urinary tract system (kidney and ureter) or the lower urinary tract system (the bladder and the urethra) of both males and females.
LABORATORY DIAGNOSIS OF ESCHERICHIA COLI INFECTION
The laboratory diagnoses of E. coli-associated diarrheal diseases are mainly based on microscopy and isolation/culture of the infecting pathogen from clinically important specimens. Blood, urine, stool, and pus are some of the specimens obtained for laboratory analysis. A mid-stream urine (MSU) is required to diagnose UTI caused by E. coli in the laboratory. Bacterial counts of E. coli less than 103 colony forming unit (CFU) per ml of urine indicate contamination while counts above 105 CFU/ml of urine are a strong indication of E. coli infection (i.e. significant bacteriuria). E. coli grow on MacConkey agar to produce smooth pink colonies, blood agar (to produce mucoid and haemolytic colonies for some strains), and on cystein lactose electrolyte deficient (CLED) medium (to produce yellow colonies). Molecular detection techniques for prompt identification of E. coli strains include the use of PCR and DNA probes.
E. coli growing on blood agar.
TREATMENT OF ESCHERICHIA COLI INFECTION
Most cases of gastroenteritis caused by bacteria in the Enterobacteriaceae family (E. coli in particular) are self-limiting and often heal without any antibacterial therapy. Since E. coli-associated diarrheal diseases are often accompanied with the loss of fluids and electrolytes from the body, treatment and management of the disease should be started with fluid and electrolyte replacement. Administration of oral-rehydration therapy (containing specific amount of salt, sugar and water) to counter the possible dehydration in the diarrhea patient is the best form of supportive therapy because it returns the affected individual to a normal fluid and electrolyte of the body.
PREVENTION AND CONTROL OF ESCHERICHIA COLI INFECTION
The presence of E. coli in drinking water and water meant for other domestic or industrial purposes is enough indication of feacal contamination of the water source from either sewage or human feaces (especially in places where people defecate in water). Food meant for human consumption becomes contaminated with the pathogen when such water sources are used for food processing. Thus, the prevention of E. coli-associated diarrheal diseases involves avoiding the consumption of contaminated water and food. Travelers or tourists visiting tropical countries should ensure that they only eat properly cooked food and drink only bottled and well disinfected water.
Prescott L.M., Harley J.P and Klein D.A (2005). Microbiology. 6th ed. McGraw Hill Publishers, USA.
Madigan M.T., Martinko J.M., Dunlap P.V and Clark D.P (2009). Brock Biology of Microorganisms, 12th edition. Pearson Benjamin Cummings Inc, USA.
Balows A, Hausler W, Herrmann K.L, Isenberg H.D and Shadomy H.J (1991). Manual of clinical microbiology. 5th ed. American Society of Microbiology Press, USA.
Barrett J.T (1998). Microbiology and Immunology Concepts. Philadelphia, PA: Lippincott-Raven Publishers. USA.
Basic laboratory procedures in clinical bacteriology. World Health Organization (WHO), 1991. Available from WHO publications, 1211 Geneva, 27-Switzerland.
Murray P.R, Baron E.J, Jorgensen J.H., Pfaller M.A and Yolken R.H (2003). Manual of Clinical Microbiology. 8th edition. Volume 1. American Society of Microbiology (ASM) Press, Washington, D.C, U.S.A.
Murray P.R, Baron E.J, Jorgensen J.H., Pfaller M.A and Yolken R.H (2003). Manual of Clinical Microbiology. 8th edition. Volume 2. American Society of Microbiology (ASM) Press, Washington, D.C, U.S.A.
Murray P.R., Rosenthal K.S., Kobayashi G.S., Pfaller M. A. (2002). Medical Microbiology. 4th edition. Mosby Publishers, Chile.