Shigellae, Salmonellae & Proteus
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THE SHIGELLAE
The natural habitat of shigellae is limited to the intestinal tracts of humans, where
they produce bacillary dysentery.
Morphology & Identification
TYPICAL ORGANISMS : - Shigellae are slender gram-negative rods.
CULTURE :- Shigellae are facultative anaerobes but grow best aerobically.
GROWTH CHARACTERISTICS
All shigellae ferment glucose. With the exception of Shigella sonnei, they do not
ferment lactose. The inability to ferment lactose distinguishes shigellae on
differential media. Shigellae form acid from carbohydrates but rarely produce gas.
They may also be divided into those that ferment mannitol and those that do not
(Table 16–3).
Table 16–3. Pathogenic Species of Shigella.
Present Designation Group and Type Mannitol Ornithine Decarboxylase
S dysenteriae
A
-
-
S flexneri
B
+
-
S boydii
C
+
-
S sonnei
D
+
+
Antigenic Structure
Shigellae have a complex antigenic pattern.
The somatic O antigens of shigellae are lipopolysaccharides. There are more than
40 serotypes. The classification of shigellae relies on biochemical and antigenic
characteristics. The pathogenic species are shown in Table 16–3.
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Pathogenesis & Pathology
- Shigella infections are almost always limited to the gastrointestinal tract;
bloodstream invasion is rare.
- Shigellae are highly communicable; the infective dose is on the order of 103
organisms (whereas it usually is 105–108 for salmonellae and vibrios).
- invasion of the mucosal epithelial cells (eg, M cells) by induced
phagocytosis, escape from the phagocytic vacuole, multiplication and spread
within the epithelial cell cytoplasm, and passage to adjacent cells.
- Microabscesses in the wall of the large intestine and terminal ileum lead to
necrosis of the mucous membrane.
- Superficial ulceration, bleeding, and formation of a "pseudomembrane"
on the ulcerated area. This consists of fibrin, leukocytes, cell debris, a
necrotic mucous membrane, and bacteria. As the process subsides,
granulation tissue fills the ulcers and scar tissue forms.
Toxins
1. (endotoxins)
Upon autolysis, all shigellae release their toxic lipopolysaccharide. This
endotoxin probably contributes to the irritation of the bowel wall.
2. Shigella dysenteriae (exotoxins)
- S dysenteriae type 1 (Shiga bacillus) produces a heat-labile exotoxin that
affects both the gut and the central nervous system. The exotoxin is a
protein that is antigenic (stimulating production of antitoxin) and lethal
for experimental animals.
- Acting as an enterotoxin, it produces diarrhea. In humans, the exotoxin also
inhibits sugar and amino acid absorption in the small intestine.
- Acting as a "neurotoxin," this material may contribute to the extreme
severity and fatal nature of S dysenteriae infections and to the central
nervous system reactions observed in them (ie, meningismus, coma).
-
The two may act in sequence, the toxin producing an early nonbloody,
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voluminous diarrhea and the invasion of the large intestine resulting in later
dysentery with blood and pus in stools.
Clinical Findings
- After a short incubation period (1–2 days),
- there is a sudden onset of abdominal pain, fever, and watery diarrhea.
- A day or so later, as the infection involves the ileum and colon, the number of
stools increases; they are less liquid but often contain mucus and blood.
- Each bowel movement is accompanied by straining and tenesmus (rectal
spasms), with resulting lower abdominal pain.
- In more than half of adult cases, fever and diarrhea subside spontaneously in 2–
5 days.
- in children and the elderly, loss of water and electrolytes may lead to
dehydration, acidosis, and even death.
- On recovery, most persons shed dysentery bacilli for only a short period, but a
few remain chronic intestinal carriers and may have recurrent bouts of the
disease.
- Upon recovery from the infection, most persons develop circulating
antibodies to shigellae, but these do not protect against reinfection.
Diagnostic Laboratory Tests
SPECIMENS
Specimens include fresh stool, mucus flecks, and rectal swabs for culture. Large
numbers of leukocytes and red blood cells are seen microscopically. Serum
specimens, if desired, must be taken 10 days apart to demonstrate a rise in titer of
agglutinating antibodies.
CULTURE
The materials are streaked on differential media (eg, MacConkey's or EMB agar)
and on selective media (Hektoen enteric agar or salmonella-shigella agar).
Colorless (lactose-negative) colonies are inoculated into triple sugar iron agar.
Organisms that fail to produce H2S, that produce acid but not gas in the butt and an
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alkaline slant in triple sugar iron agar medium, and that are nonmotile should be
subjected to slide agglutination by specific shigella antisera.
SEROLOGY : - Serology is not used to diagnose shigella infections.
Immunity
- Infection is followed by a type-specific antibody response.
- Injection of killed shigellae stimulates production of antibodies in serum but
fails to protect humans against infection.
- IgA antibodies in the gut may be important in limiting reinfection; these may be
stimulated by live attenuated strains given orally as experimental vaccines.
- Serum antibodies to somatic shigella antigens are IgM.
Treatment
- Ciprofloxacin, ampicillin, doxycycline, and trimethoprim-sulfamethoxazole
are most commonly inhibitory for shigella.
- They may fail to eradicate the organisms from the intestinal tract. Multiple drug
resistance can be transmitted by plasmids, and resistant infections are
widespread.
- Many cases are self-limited.
- Opioids should be avoided in shigella dysentery.
Epidemiology, Prevention, & Control
- Shigellae are transmitted by "food, fingers, feces, and flies" from person to
person. Most cases of shigella infection occur in children under 10 years of
age.
- Since humans are the main recognized host of pathogenic shigellae, control
efforts must be directed at eliminating the organisms from this reservoir by (1)
sanitary control of water, food, and milk; sewage disposal; and fly control; (2)
isolation of patients and disinfection of excreta; (3) detection of subclinical
cases and carriers, particularly food handlers; and (4) antibiotic treatment of
infected individuals.
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THE SALMONELLA-ARIZONA GROUP
- Salmonellae are often pathogenic for humans or animals when acquired by the
oral route.
- They are transmitted from animals and animal products to humans, where they
cause enteritis, systemic infection, and enteric fever.
- Salmonellae isolates are motile.
- Salmonellae grow readily on simple media,
- They survive freezing in water for long periods.
- Salmonellae are resistant to certain chemicals (eg, sodium tetrathionate,
sodium deoxycholate).
Classification
- The members of the genus Salmonella were originally classified on the basis of
epidemiology, host range, biochemical reactions, and structures of the O,
H, and Vi (when present) antigens.
- DNA-DNA hybridization studies have demonstrated that there are seven
evolutionary groups. Nearly all of the salmonella serotypes that infect humans
are in DNA hybridization group I . (Table 16–4).
Table 16–4. Representative Antigenic Formulas of Salmonellae.
O Group
Serotype
Antigenic
D
S Typhi
9, 12 (Vi):d:—
A
S Paratyphi A
1, 2, 12:a—
C1
S Choleraesuis
6, 7:c:1,5
B
S Typhimurium
1, 4, 5, 12:i:1, 2
D
S Enteritidis
1, 9, 12:g, m:—
1
O antigens: boldface numerals.
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(Vi): Vi antigen if present.
Phase 1 H antigen: lower-case letter.
Phase 2 H antigen: numeral.
Pathogenesis & Clinical Findings
- Salmonella Typhi, Salmonella Choleraesuis, and perhaps Salmonella
Paratyphi A and Salmonella Paratyphi B are primarily infective for humans,
and infection with these organisms implies acquisition from a human source.
- The vast majority of salmonellae, however, are chiefly pathogenic in animals
that constitute the reservoir for human infection: poultry, pigs, rodents, cattle,
pets.
- The organisms enter via the oral route, usually with contaminated food or
drink.
- The mean infective dose to produce clinical or subclinical infection in humans
is 105–108 salmonellae. Among the host factors that contribute to resistance to
salmonella infection are gastric acidity, normal intestinal microbial flora,
and local intestinal immunity.
- Salmonellae produce three main types of disease in humans, but mixed forms
are frequent.
1. THE "ENTERIC FEVERS" (TYPHOID FEVER)
- This syndrome is produced by only Salmonella Typhi (typhoid fever).
- salmonellae reach the small intestine, from which they enter the lymphatics
and then the bloodstream. They are carried by the blood to many organs. The
organisms multiply in intestinal lymphoid tissue and are excreted in stools.
- After an incubation period of 10–14 days, fever, malaise, headache,
constipation, bradycardia, and myalgia occur. The fever rises to a high
plateau, and the spleen and liver become enlarged. Rose spots, usually on the
skin of the abdomen or chest, are seen briefly in rare cases. In the preantibiotic
era, the chief complications of enteric fever were intestinal hemorrhage and
perforation, and the mortality rate was 10–15%. Treatment with antibiotics has
reduced the mortality rate to less than 1%.
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- The principal lesions are hyperplasia and necrosis of lymphoid tissue (eg,
Peyer's patches), hepatitis, focal necrosis of the liver, and inflammation of
the gallbladder, periosteum, lungs, and other organs.
2. BACTEREMIA WITH FOCAL LESIONS
- This is associated commonly with S choleraesuis but may be caused by any
salmonella serotype.
- Following oral infection, there is early invasion of the bloodstream (with
possible focal lesions in lungs, bones, meninges, etc), but intestinal
manifestations are often absent. Blood cultures are positive.
3. ENTEROCOLITIS
- This is the most common manifestation of salmonella infection.
- Salmonella Typhimurium and Salmonella Enteritidis are prominent, but
enterocolitis can be caused by any of the group I serotypes of salmonellae.
Eight to 48 hours after ingestion of salmonellae, there is nausea, headache,
vomiting, and profuse diarrhea. Low-grade fever is common. Inflammatory
lesions of the small and large intestine are present. Bacteremia is rare (2–4%)
except in immunodeficient persons. Blood cultures are negative, but stool
cultures are positive.
Diagnostic Laboratory Tests
SPECIMENS
- Blood for culture must be taken repeatedly. In enteric fevers and septicemias,
blood cultures are often positive in the first week of the disease.
- Urine cultures may be positive after the second week.
- Stool specimens also must be taken repeatedly. In enteric fevers, the stools
yield positive results from the second or third week on; in enterocolitis, during
the first week.
ISOLATION OF SALMONELLAE
1. BACTERIOLOGIC METHODS
2. SEROLOGIC METHODS
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Serologic techniques are used to identify unknown cultures with known sera and
may also be used to determine antibody titers in patients with unknown illness.
Agglutination Test
In this test, known sera and unknown culture are mixed on a slide. Clumping,
when it occurs can be observed within a few minutes. This test is particularly
useful for rapid preliminary identification of cultures. There are commercial kits
available to agglutinate and serogroup salmonellae by their O antigens: A, B, C 1,
C2, D, and E.
Tube Dilution Agglutination Test (Widal Test)
Serum agglutinins rise sharply during the second and third weeks of Salmonella
Typhi infection. The Widal test to detect these antibodies against the O and H
antigens has been in use for decades. At least two serum specimens, obtained at
intervals of 7–10 days, are needed to prove a rise in antibody titer. Serial dilutions
of unknown sera are tested against antigens from representative salmonellae.
False-positive and false-negative results occur. The interpretive criteria when
single serum specimens are tested vary, but a titer against the O antigen of >
1:320 and against the H antigen of > 1:640 is considered positive. High titer of
antibody to the Vi antigen occurs in some carriers. Results of serologic tests for
salmonella infection must be interpreted cautiously because the possible presence
of cross-reactive antibodies limits the use of serology. The test is not useful in
diagnosis of enteric fevers caused by salmonella other than Salmonella Typhi.
Immunity
- Infections with Salmonella Typhi or Salmonella Paratyphi usually confer a
certain degree of immunity.
- Reinfection may occur.
- Circulating antibodies to O and Vi are related to resistance to infection and
disease.
- Secretory IgA antibodies may prevent attachment of salmonellae to intestinal
epithelium.
Treatment
- In severe diarrhea, replacement of fluids and electrolytes is essential.
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- Antimicrobial therapy of invasive salmonella infections is with ampicillin,
trimethoprim-sulfamethoxazole, or a third-generation cephalosporin.
- Multiple drug resistance transmitted genetically by plasmids among enteric
bacteria is a problem in salmonella infections. Susceptibility testing is an
important adjunct to selecting a proper antibiotic.
- In most carriers, the organisms persist in the gallbladder (particularly if
gallstones are present) and in the biliary tract.
- Some chronic carriers have been cured by ampicillin alone, but in most cases
cholecystectomy must be combined with drug treatment.
Prevention & Control
- Sanitary measures .
- Infected poultry, meats, and eggs must be thoroughly cooked.
- Carriers must not be allowed to work as food handlers.
- Two injections of acetone-killed bacterial suspensions of Salmonella Typhi,
followed by a booster injection some months later, give partial resistance to
small infectious inocula of typhoid bacilli but not to large ones.
- Oral administration of a live avirulent mutant strain of Salmonella Typhi
has given significant protection in areas of high endemicity.
- Vaccines against other salmonellae give less protection and are not
recommended.
Epidemiology
- The feces of persons who have unsuspected subclinical disease or are carriers
are a more important source of contamination.
- Many animals, including cattle, rodents, and fowl, are naturally infected with a
variety of salmonellae and have the bacteria in their tissues (meat), excreta, or
eggs.
- The high incidence of salmonellae in commercially prepared chickens has been
widely publicized.
- The problem probably is aggravated by the widespread use of animal feeds
containing antimicrobial drugs that favor the proliferation of drug-resistant
salmonellae and their potential transmission to humans.
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CARRIERS
After manifest or subclinical infection, some individuals continue to harbor
salmonellae in their tissues for variable lengths of time .Three percent of survivors
of typhoid become permanent carriers, harboring the organisms in the gallbladder,
biliary tract, or, rarely, the intestine or urinary tract.
SOURCES OF INFECTION
Water, Milk and Dairy Products (Ice Cream, Cheese, Custard),Shellfish and Dried
or Frozen Eggs.
Proteus is a genus of Enterobacteriacae, widely distributed in nature as
saprophytes, being found in decomposing animal matter, in sewage, in manure soil,
and in human and animal feces. They are opportunistic pathogens, commonly
responsible for urinary and septic infections, often nosocomial.
Clinical significance
Three species—P. vulgaris, P. mirabilis, and P. penneri—are opportunistic human
pathogens. Proteus includes pathogens responsible for many human urinary tract
infections.
P. mirabilis is often found as a free-living organism in soil and water.causes
wound and urinary tract infections. Most strains of P. mirabilis are sensitive to
ampicillin and cephalosporins.
P. vulgaris is isolated less often in the laboratory and usually only targets
immunosuppressed individuals. P. vulgaris occurs naturally in the intestines of
humans and a wide variety of animals, and in manure, soil, and polluted waters.
P. mirabilis, once attached to the urinary tract, infects the kidney more commonly
than E. coli. About 10-15% of kidney stones are struvite stones, caused by
alkalinization of the urine by the action of the urease enzyme (which splits urea
into ammonia and carbon dioxide) of Proteus (and other) bacterial spp.
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