Escherichia coli infections

C L GYLES AND M M HENTON

GENERAL INTRODUCTION: FACULTATIVELY ANAEROBIC GRAM-NEGATIVE RODS

Introduction

Escherichia coli predominates among the facultatively anaerobic bacteria in the intestine of many animal species. ¹⁶⁸ Most E. coli strains are nonpathogenic but several subsets of the species have become pathogenic through the acquisition of genes that encode virulence factors, many of which are encoded on extrachromosomal genetic elements, temperate bacteriophages, or large transmissible blocks of genes called pathogenicity islands.^{37, 61, 73, 74, 111, 114, 115} Escherichia coli infections in livestock may be enteric or extraintestinal. The enteric infections involve colonization of the intestine and the action of toxins and/or signal transduction events. Enteric colibacillosis occurs most commonly in new-born animals in intensive farming systems, but only rarely in animals raised under extensive conditions. The most common clinical manifestation of the enteric infections is diarrhoea. Extraintestinal infections include septicaemia, oedema disease, mastitis, uterine infection, and abortion.

Escherichia coli infections occur worldwide,^{2, 23, 69, 75, 85, 92, 187, 189, 191} and can be of major economic significance. In southern Africa, enteric colibacillosis occurs most commonly in piglets, and less frequently in calves, lambs and kids.^{75, 76}

Aetiology

Escherichia coli is a Gram-negative, straight, usually motile rod 1,1 to 1,5μm wide and 2 to 6 μm long, that belongs to the Enterobacteriaceae.^{16, 32, 145} It is facultatively anaerobic, having both respiratory and fermentative types of metabolism, and some strains possess peritrichous flagellae.

The organisms form large, pink colonies on MacConkey agar and are typically indole positive, and urease, citrate and hydrogen sulfide negative.¹⁶ Most colonies on nutrient agar are either smooth or rough, but intermediate forms and mucoid colonies do occur. Smooth forms are low-convex and shiny, possess an entire, regular circumference, and are easily dispersible in saline, while rough colonies are dry, have a dull appearance, are usually flattish, have an irregular outline, and are difficult to disperse in saline.

As a result of mutation, rough forms have lost the polysaccharide side chains of the lipopolysaccharide of the outer membrane of the cell wall. They autoagglutinate in saline and are therefore impossible to serotype by conventional means.

Escherichia coli is distinguishable from Citrobacter in that citrate cannot be used as its sole carbon source. Lactose is fermented by most strains of E. coli, and the colonies are therefore pink on MacConkey agar. Escherichia coli is usually a strong indole producer, which distinguishes it from Enterobacter. Unlike Klebsiella, E. coli does not ferment inositol or adonitol.¹⁶

An international serotyping scheme has been developed on the basis of O (the polysaccharide portion of the cell wall lipopolysaccharide), K (capsular polysaccharide), and H (flagellar protein) antigens.^{108, 146} The O antigens have been assigned numbers from 1 to 173, the K antigens 1 to 103, and the H antigens 1 to 50. If only the ‘O’ antigen of an isolate is identified it is said to belong to a particular ‘O’ serogroup. Isolates with the same serotype usually have many features in common but may have important differences as well. Protein appendages called pili or fimbriae constitute a fourth type of surface antigen which is often used in characterization of pathogenic E. coli.^{8, 37, 57, 105, 125, 131} Many of these are critical virulence factors which allow the E. coli to bind to specific pilus receptors on the surface of host epithelial cells in the intestine or urinary bladder. The fimbrial antigens have ‘F’ designations but other names which preceded the F designation are sometimes used.

Because E. coli are normal enteric flora and also enteric pathogens of livestock, steps need to be taken to assess whether an isolate from the faeces or intestine of an animal with diarrhoea or dysentery is a pathogen. Serotype is often a strong indicator of pathogenicity,^{72, 108, 146} but serotyping can be done in only a limited number of laboratories that have the appropriate complete collection of specific absorbed antisera and the relevant control strains. The best indicator of pathogenicicty is the possession of virulence factors such as enterotoxins, cytotoxins, colonization pili, and adhesins.^{1, 7, 8, 19, 37, 39, 57, 71, 73, 131, 136, 164}

The term pathotype is used to describe a group of E. coli with the same array of virulence factors. There may be different pathotypes within a serotype or a single pathotype in several serotypes. Other subtyping schemes that may be used for comparing E. coli isolates include biotyping, phage typing, plasmid profiling, and pulsed field gel electrophoresis.^{16, 77, 103, 108}

The types of E. coli that are responsible for enteric diseases in livestock are enterotoxigenic E. coli (ETEC), enteropathogenic E. coli (EPEC), and Shiga toxin-producing E. coli (STEC) (Table 150.1).^{106, 108, 137} An additional category of E. coli not listed in Table 150.1 and suggested to be diarrhoeagenic in cattle is a group of E. coli which produce cytotoxic necrotizing factor (CNF) and are referred to as necrotizing E. coli (NTEC).^{39, 40}

Enterotoxigenic E. coli (ETEC)

Escherichia coli in this category produce one or more enterotoxins and colonize the small intestine...