E. Coli

Escherichia coli was so named after Theodore Escherich due to his pioneering studies on the fecal flora in neonates. He described the organism in 1882. Further, E.coli was classified under the genus Escherichia in the family Enterobactericeae.   

Spectrum of diseases caused by E.coli

In humans, E.coli is known to cause a variety of infections ranging from diarrheas and urinary tract infections to hospital acquired, or nosocomial infections attacking wounds and the respiratory tract.

  • Diarrheas: There are six different pathotypes of E.coli causing diarrheas in human beings.
    • Enterotoxigenic E.coli (ETEC): This organism is largely responsible for traveler’s diarrhea and diarrheal episodes in children in the developing countries. It presents as watery diarrhea which can cause severe dehydration at times. The spread is primarily via contaminated food and water. The standard for individuals traveling in developing countries should be ‘boil it, cook it, peel it and forget it’. Antimotility agents like Loperamide and fluoroquinolones are the principal agents utilized in the treatment of ETEC induced diarrheas.
    • Enteropathogenic E.coli (EPEC): This pathotype is accountable for infantile diarrhea in children from developing countries. The patient presents with severe acute diarrhea accompanied by persistent vomiting. It is often a self-limiting disease but may need antimicrobial therapy if severe. Treatment should be determined by the susceptibility pattern. The transmission is primarily from person to person. Improving standards of hygiene and creating provisions for clean, safe drinking water is the most critical measure in bringing about outbreaks under control.
    • Enterohemorrhagic E.coli (EHEC): Initially the patient presents with watery stools followed by bloody diarrhea. EHEC is the most common cause of invasive bloody diarrhea in developing countries. The most dangerous complication of this pathotype is the development of hemolytic uremic syndrome. This syndrome is a triad of hemolytic anemia, thrombocytopenia and uremia. EHEC produce certain enterotoxins called Shiga toxins which have an affinity for the renal glomerular cells and the platelets causing HUS. The Shiga toxin is also known as the Verocytotoxin as its toxicity features have been observed and studied in Vero cell line cultures. It is notable that certain drugs such as co-trimoxazole, furadantin and fluoroquinolones are prone to induce the production of this toxin. Therefore, in case of suspicion, antimicrobial therapy is at best restricted. HUS and diarrheas due to EHEC are likely to occur in due to consumption of contaminated foods, primarily meat and meat products. The E.coli O 157 : H7 strain has been classically associated with outbreaks of EHEC leading to the development of  HUS. Apart from serotyping, mannitol fermentation is a critical characteristic which can help with the differentiation of these strains from fecal specimens. 
    • Enteroaggregative E.coli (EAEC): It is the cause of chronic, persistent diarrhea among children and traveler’s in developing countries. The diarrhea is mucoid in nature and responds well to fluoroquinolones.
    • Enteroinvasive E.coli (EIEC):  It causes outbreaks of diarrhea and dysentery in developed countries, primarily due to consumption of contaminated food.
    • Diffuse adhering E.coli (DAEC): This pathotype is the causative agent of diarrhea in older children in developing countries. This type has been recently added and more is yet to be learned about its pathogenicity and mode of transmission.
  • Urinary tract Infections: E.coli is the major cause of community acquired urinary tract infections in women. They are also referred to as  Uropathogenic E.coli (UPEC). Uropathogenic E.coli produce the P and S fimbriae that bind to the glycolipid receptors on the surface of the host cells. P and S fimbriae are virulence factors for uropathogenic E.coli. The serotypes isolated from the urine cultures often belong to O group 1,2,4,6,7 etc , present in the fecal flora. Other factors which determine the pathogenicity of uropathogenic E.coli (UPEC) are:
    • their ability to produce a capsule 
    • to produce a toxin which is a cytolytic hemolysin 
    • to have multiple mechanisms for the uptake of iron Strains of E.coli isolated from urine cultures are different from those isolated from cases of diarrhea. Uropathogenic E.coli that are community acquired are likely to be susceptible to the common drugs though resistance to fluoroquinolones is now being seen among these strains. The hospital acquired UPEC are likely to be multi-drug resistant and often colonize urinary catheters thereby becoming potential pathogens. These strains are more like to be multi-drug resistant and therefore pose a problem in treatment. Another critical feature noted among E.coli organisms is that individuals with compromised immune systems or structural abnormalities of the genitourinary tract are more likely to be infected due to control strains being isolated from the fecal flora.
  • Wound Infections: E.coli is the leading cause of wound infections among the gram negative rods. The majority of these strains are likely to be multi-drug resistant. Such strains are a concern to the infection control practitioners d thatue to the fact they are likely to spread from one patient to another via contaminated hands of healthcare workers (HCWs).
  • Meningitis: E.coli induced meningitis is common among neonates, especially if there is evidence of a leak per vaginum before delivery. Traditionally, they are second only to Streptococcus agalactiae in causing neonatal meningitis.  These strains are similar to the UPEC strains which produce the S fimbriae and the K1 capsule. Clonal similarity has also been observed among the strains of neonatal meningitis and those causing urinary tract infections as well as other extraintestinal infections.  

Drug resistance among E.coli

E.coli has histrically produced betalactamases which are capable of hydrolyzing the penicillin group of drugs. The 1960s saw the emergence of plasmid mediated Extended Spectrum betalactamases (ESBLs) from E.coli isolates. These are capable of hydrolyzing not only penicillins, but also cephalosporins and monobactums. Cephalosporins and monobactums are the first line drugs utilized for hospitalized patients until the susceptibility report has been recieved. Additionally, it has been noted that ESBL producing isolates are likely to carry resistance genes to the aminoglycoside group of antimicrobials and also to the fluoroquinolones. This makes these strains multi-drug resistant. ESBL producing E.coli is susceptible to Betalactam and Betalactamase inhibitor combinations such as carbapenams. Another class of betalactamases called Amp C (Cephalosporinases) produced by E.coli are resistant to cephalosporins besides being resistant to Betalactam and Betalactamase inhibitor combinations. They are nonetheless susceptible to carbapenams. A new clinically significant resistance has recently been encountered with E.coli which is capable of hydrolyzing even carbapenams. These have been christened metallobetalactamase (MBL) producers. There has been mounting evidence of E.coli strains producing all three classes of enzymes which has made treatment of infections caused by such strains extremely difficult due to the fact that they are unresponsive to antimicrobial drugs currently available. The majority of these drug resistant strains are hospital acquired and can be brought under control by following strict infection control guidelines and utilizing an antibiotic policy based on the susceptibility patterns in a particular healthcare setting.