Literature Review - Part 2

1.8 Salmonella

Salmonella belongs to the family Enterobacteriaceae, a group of bacteria that inhabit the intestinal tracts of humans and other warm blooded animals. The typhoid bacillus was first isolated in 1880 by Eberth from the mesenteric glands and spleen of people dying from typhoid fever.

The members of this genus are Gram negative rods, and are facultatively anaerobic. Colonies formed on agar have irregular “Maple leaf” margines and slightly roughened glistening surfaces. The Salmonellae have simple nutritional requirements and stain readily with the usual dyes, such as methylene blue and carbolfuchsin. All species except S. pullorum and S. gallinarum are actively motile by means of peritrichious flagella. Their optimum growth temperature is 37˚C, but grow reasonably well at room temperature. They are also characterized biochemically by their failure to ferment lactose or salicilin, and their inability to liquefy gelatin or produce indole.

Almost all members of the genus Salmonella are potentially pathogenic. Because of this there are extensive biochemical and serological tests to clinically isolate and identify them. They are by default common inhabitants of animal intestinal tracts, especially poultry and cattle, and under unsanitary conditions, they can contaminate food (Tortora et al 1998).

Salmonellae rarely form toxins; however, all contain endotoxins – the polysaccharide-polypeptide-lipid complexes found in the outer membrane. The toxicity is nonspecific because all endotoxins, regardless of source, produce approximately the same reactions on parenteral inoculation. Among the responses generated are fever and alteration in capillary permeability. Since these endotoxins are antigenic, they stimulate the formation of agglutinating, precipitating and protective antibody activity; however, the anti-toxic activity of these antibodies is of a low order.

1.9 Nomenclature

There is yet to be a satisfactory nomenclature of Salmonella, although the current method of nomenclature considers Salmonella to be a single species divided into roughly 2000 serovars or serotypes which are differentiated by serological techniques. The Centre for Disease Control and Prevention (CDC) has adopted, for practical clinical purposes, a system in which three species are recognized: S. typhi, S. choleraesuis and S. enteritidis. All other salmonellae are defined as serotypes of the species S. enteritidis, but because a name like Salmonella enteritidis serotype typhimurium is long and awkward, Salmonella typhimurium (which sounds like a species name) is more commonly used.

1.10 Salmonella antigens

Salmonella serovars are distinguished by serological means. This is done by injecting the bacteria into appropriate animals and thereby allowing their flagella, capsules and cell walls to cause the production of antibodies specific for these antigens by the animal’s immune system. The Kauffmann-White scheme dictates these serovars by designating to an organism letters and numbers corresponding to specific antigens on the organism’s capsule , flagella and cell wall . The O antigens are heat stable polysaccharides that form part of the cell wall lipopolysaccharide. These surface polysaccharides inhibit the agglutinability of Salmonella by homologous antisera. O polysaccharides have a core structure to which are attached side chains of sugars, and it has been determined that these side chains dictate the bacteria’s specificity. The H antigens, on the other hand, are heat labile proteins of the flagella, and are distinctly unique in diphasic variation.

1.11 Salmonella infections

Salmonella infects both animals and human beings. Salmonella infection of rodents is quite common, the bacteria involved being S. typhimurium and S. enteriditis. These animals may become healthy carriers, and are associated with the epidemiology of food poisoning outbreaks. Dogs, too, may be infected; an incidence as high as 27% has been observed. Its importance as a reservoir of human infection is unclear; however, dog-to-human transmissions have been documented. Horses and birds are also susceptible to infection, and lately, chickens infected with Salmonella have been found to spread the disease through trans-ovarian transfer to its eggs, which are then consumed by the human population.

Ordinarily, cold-blooded animals are not reservoirs of infection related to the infection of humans. However, a number of human infections have been found to have been acquired from infected pet turtles. These may acquire the infection from penetration of the unhatched turtle eggs by Salmonella (Freeley & Treger, 1969), as well as from other turtles or infected water. Freshwater aquarium snails have also been found to harbour Salmonella, along with other potentially pathogenic, Gram-negative bacilli.

1.12 Salmonella typhi and typhoid fever

Salmonella typhi is the human pathogen responsible for the disease called typhoid fever. This disease is acquired by ingesting food or water that has been contaminated, either by human faeces directly (such as sewage-contaminated water supplies), or by handlers of food who are carriers and shed large numbers of bacteria in their faecal wastes.

The incubation time for S. typhi ranges from 1 week to 1 month, and its infective dose is roughly 100,000 cells. Upon ingestion of this bacteria, S. typhi enters the body through M cells, sometimes causing ulceration of the intestine. When S. typhi binds to the host cell wall, it causes a change in the appearance of the cell surface called “ruffling” and the rearrangement of actin, which results in the internalization of the bacteria inside an endocytic vesicle. It them proceeds to multiply in the submucosal layer and in the lymphatic system (the intestinal lymph follicles and draining mesenteric lymph nodes; the characteristic lesions of typhoid fever are found in these regions). It enters the bloodstream via the thoraxic lymph, bringing about the condition called bacteremia, and spreads to other parts of the body. Those which multiply in the spleen and liver are then released into the bloodstream in great numbers. This stage of the disease may last up to 2 or 3 weeks, and is manifested as high fever, flushing and anorexia, and sometimes chills, convulsions, delirium and abdominal pain that is either diffuse or localized to the right lower quadrant over the terminal ileum, a symptom mimicking appendicitis. The fever symptoms are probably caused by the LPS-mediated release of cytokines. It is known that a part of the brain called the hypothalamus controls the body temperature. S. typhi possesses an antigen called the Vi antigen, this being a polysaccharide capsule surrounding the O antigen, thus protecting the bacteria from antibody attacks on the O antigen. When phagocytes ingest S. typhi, this polysaccharide antigen is released, and this causes the phagocyte to release a cytokine called interleukin-1 (IL-1), which causes the hypothalamus to release prostaglandins. This resets the hypothalamic thermostat at a higher temperature, and causes fever. Inflammation of the spleen may also occur (a condition called splenomegaly) and the patient may also develop diarrhoea and rose-coloured spots on the abdomen, this being a transient rash and only seen on light-skinned people (Gladwin and Trattler, 1997).

The bacteria then move from the liver to the gallbladder and are shed in bile into the intestines for the second time, sometimes causing severe ulceration. The disease may also be accompanied by disseminated intravascular coagulation, a condition in which platelets and coagulation factors are decreased, leading to haemorrhage. If not treated by this stage, the victim will most likely die.

The immune response against this bacteria is largely cell-mediated. However, strains of S. typhi resistant to reactive forms of oxygen and defensins have proven to be capable of living inside phagocytes, since they are able to survive the phagocyte killing mechanism. There is also humoral response because agglutinins, precipitations and complement-dependent bactericidal antibodies are also formed.

Salmonella typhi may be cultured within the first 10 days in the majority of cases. The presence of this bacteria in the blood, however, does not constitute septicaemia. They are also present in the bone marrow early in the disease. Serum agglutinins appear as early as the fifth day of infection, and by four weeks, are present in 90% of the cases. In general, O agglutinin titers of 1:100 and H titers of 1:200 are considered indicative of infection, especially if increases in titer are noted with time. During and after the second week, S. typhi may be found with increasing frequency in the faeces (Burrows).

Typhoid fever may also pave the way for other complications, such as laryngeal ulcer and cystisis (when the gallbladder is infected). Suppurative and inflammatory processes may appear in other parts of the body. Diseases of the periosteum, bone marrow and joints have also been associated with infections by S. typhi. In addition, osteomyelitis may also develop as long as 6 or 7 years following the recovery from typhoid fever, showing that this bacillus can remain in contact with human tissues for many years without losing its virulence (Burrows).

1.13 Epidemiology of Salmonella typhi

Some S. typhi strains are more virulent than others, but all of them cause illness that can be fatal if left untreated. About 10% of those who are infected succumb to the disease. However, there are also those who have been left entirely unharmed by the bacteria. Those who have been infected and have recovered usually become carriers. This constitutes about one-third of the individuals infected, and they may discharge the bacteria for a period of three weeks after the onset of the illness; 10% shed the bacteria for 8 to 10 weeks. This carrier state is usually due to persistence of infection in the gallbladder. The urinary bladder may also be infected, resulting in urinary carriers who excrete the bacteria in the urine; however, this state is uncommon, except in countries such as Egypt, where its development may be favoured by a pathological condition of the urinary tract associated with schistosomiasis (Bassily et al, 1972). Most of these carriers are women, although the reason for this is unknown. The percentage of patients becoming carriers seems to be age related too – a carrier rate as high as 10.1% has been recorded for the 50-59 age group. As these carriers excrete the bacteria into the environment, this has posed a problem in the food industry, as these carriers are perfectly capable of spreading the disease to naïve individuals; however, adequate sanitation and stringent regulations dictating that all these food handlers wash their hands properly and regularly has lowered the incidence of this disease in developed countries. However, this disease still persists as a large epidemic foci in underdeveloped countries, and as sporadic cases and small outbreaks in smaller towns and rural areas of developed countries.

1.14 Treatment and prevention of typhoid fever

Typhoid fever victims are treated with antibiotics ciprofloxacin or ceftriaxone. Faecal carriers may also be treated by removal of their gallbladders (effective in 70% of the cases). There are currently vaccines for typhoid fever; however, these vaccines are only 70-90% effective. The vaccine comes in two forms: oral (protects an individual for up to 5 years but causes side effects such as allergy, nausea and rash) and injectable (ViCPS, which requires a booster shot every two years). However, prior immunization, as demonstrated in the Aberdeen epidemic, does not necessarily protect individuals, and in certain cases, even appears to modify the disease. Those who have been infected once also may not be significantly protected against subsequent infection (DuPont et al, 1971). It is evident that the best prophylaxis against this disease is proper hygiene – drinking only bottled or boiled beverages and eating only thoroughly cooked food (especially eggs).