Gram -ve Enteric Tract Rods

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             Gram-negative rods are a large group of diverse organisms.

In this book, these bacteria are subdivided into three clinically relevant categories, each in a separate chapter, according to whether the organism is related primarily to the enteric or the respiratory tract or to animal sources (Table 18-1).

Gram-negative rods related to the enteric tract include a large number of genera.

These genera have therefore been divided into three groups depending on the major anatomic location of disease, namely (1) pathogens both within and outside the enteric tract, (2) pathogens primarily within the enteric tract, and (3) pathogens outside the enteric tract .(Table 18-1).

The frequency with which the organisms related to the enteric tract cause disease in the United States is shown in Table 18-2.

Salmonella, Shigella, and Campylobacter are frequent pathogens in the gastrointestinal tract, whereas Escherichia, Vibrio, and Yersinia are less so.

The medically important gram-negative rods that cause urinary tract infections are described in Table 18-4.

Patients infected with such enteric pathogens as Shigella, Salmonella, Campylobacter, and Yersinia have a high incidence of certain autoimmune diseases such as Reiter's syndrome (see Chapter 66).

Before describing the specific organisms, it is appropriate to describe the family Enterobacteriaceae.

These organisms are the major facultative anaerobes in the large intestine but are present in relatively small numbers compared with anaerobes such as Bacteroides.

Although the members of the Enterobacteriaceae are classified together taxonomically, they cause a variety of diseases with different pathogenetic mechanisms.

V Typical Bacterlologlc v,;', or Epktomlologlc Findings Ferments lactose.

P aeruginosa, a significant cause of urinary tract infection and sepsis in hospitalized patients, does not ferment glucose or reduce nitrates and is oxidase-positive.

Antigens The antigens of several members of the Enterobacteriaceae, especially Salmonella and Shigella.

Features of the Organism Colonies show metallic sheen on EMB agar.

The rationale for the use of these media and the reactions of several important organisms are presented in the Box and in Table 18-7.

In a general sense, the term "coliform" includes not only E coli but also other inhabitants of the colon such as Enterobacter and Klebsiella.

However, because only E coli is exclusively a large-intestine organism, whereas the others are found in the environment also, it is used as the indicator of fecal contamination.

In water quality testing, E coli is identified by its ability to ferment lactose with the production of acid and gas, its ability to grow at 44.5 °C, and its characteristic colony type on EMB agar.

The medium in the tube is produced such that there is a solid, poorly oxygenated area on the bottom, called the butt, and an angled, well-oxygenated area on top, called the slant.

The reactions of some of the important organisms are presented in Table 18-7.

Because several organisms can give the same reaction, TSI agar is only a screening device.

Antibiotic Therapy The appropriate treatment for infections caused by members of the Enterobacteriaceae and related organisms must be individually tailored to the antibiotic sensitivity of the organism.

Diseases E coli is the most common cause of urinary tract infection and gram-negative rod sepsis.

It is one of the two important causes of neonatal meningitis and the agent most frequently associated with ''traveler's diarrhea," a watery diarrhea.

Because there are more than 150 O, 50 H, and 90 K antigens, the various combinations result in more than 1000 antigenic types of E coli.

Pathogenesis E coli has several clearly identified components that contribute to its ability to cause disease: pili, a capsule, endotoxin, and two exotoxins (enterotoxins).

Once attached, the bacteria synthesize enterotoxins (exotoxins that act in the enteric tract), which act on the cells of the jejunum and ileum to cause diarrhea.

Enterotoxigenic strains of E coli can produce either or both of two enterotoxins.

These O157:H7 strains produce verotoxin, so called because it is toxic to Vero (monkey) cells in culture and presumably to the cells lining the colon.

These toxins are also called Shiga-like toxins because they are very similar to those produced by Shigella species.

Diseases Salmonella species cause enterocolitis, enteric fevers such as typhoid fever, and septicemia with metastatic abscesses.

This leads to bacteremia, which is associated with the onset of fever and other symptoms, probably caused by endotoxin.

Survival and growth of the organism within phagosomes in phagocytic cells are a striking feature of this disease, as is the predilection for invasion of the gallbladder, which can result in establishment of the carrier state and excretion of the bacteria in the feces for long periods.

Septicemia accounts for only about 5-10% of Salmonella infections and occurs in one of two settings: a patient with an underlying chronic disease such as sickle cell anemia or cancer or a child with enterocolitis.

Previously damaged tissues, such as infarcts and aneurysms, especially aortic aneurysms, are the most frequent sites of metastatic abscesses.

The epidemiology of Salmonella infections is related to the ingestion of food and water contaminated by human and animal wastes.

5 typhi, the cause of typhoid fever, is transmitted only by humans, but all othu-r species have a significant animal as well as human reservoir.

Human sources are either persons who temporarily excrete the organism during or shortly after an attack of enterocolitis or chronic carriers who excrete the organism for years.

Dogs and other pets, including turtles, are additional sources, Clinical Findings After an incubation period of 12-48 hours, enterocolitis begins with nausea and vomiting and then progresses to abdominal pain and diarrhea, which can vary from mild to severe, with or without blood.

The disease begins to resolve by the third week, but severe complications such as intestinal hemorrhage or perforation can occur.

Treatment Enterocolitis caused by Salmonella is usually a self-limited disease that resolves without treatment.

These two vaccines cause fewer side effects than the third vaccine, which contains killed, whole S typhi organisms.

Outbreaks occur in day-care nurseries and in mental hospitals, where fecal-oral transmission is likely to occur.

Local inflammation accompanied by ulceration occurs, but the organisms rarely penetrate through the wall or enter the bloodstream, unlike salmonellae.

Although some strains produce an enterotoxin (called Shiga toxin), invasion is the critical factor in pathogenesis.

The evidence for this is that mutants that fail to produce enterotoxin but are invasive can still cause disease, whereas noninvasive mutants are nonpathogenic.

Salmonella, or Campylobacter is involved rather than a toxin-producing organism such as V cholerae, E coli, or Clostridium perfringens.

s Diseases Campylobacter jejuni is a frequent cause of enterocolitis, especially in children.

Important Properties Campylobacters are curved, gram-negative rods that appear either comma- or S-shaped.

Food and water contaminated with animal feces is the major source of human infection.

Puppies with diarrhea are a common source for chilie dren.

Human-to-human transmission occurs but is less frequent than animal-to-human transmission.

C jejuni is a major cause of diarrhea in the United States; it was recovered in 4.6% of patients with is diarrhea, compared with 2.3% and 1% for Salmonella and Shigella, respectively.

:re The pathogenesis of both the enterocolitis and the systemic diseases is unclear.

Systemic infections, eg, bacteremia, occur most often in neonates or debilitated adults.

Gastrointestinal infection with C jejuni is associated with Guillain-Barre syndrome, the most common cause of acute neuromuscular paralysis.

Laboratory Diagnosis If the patient has diarrhea, a stool specimen is cultured on a blood agar plate containing antibiotics'' that inhibit most other fecal flora.

Identification of the organism as C intestinalis is confirmed by its failure to grow at 42 °C.

Pathogenesis & Epidemiology H p\lori attaches to the mucus-secreting cells of the gastric mucosa.

Diseases These organisms are usually opportunistic pathogens that cause nosocomial infections, especially pneumonia and urinary tract infections.

Important Properties K pneumoniae, Enterobacter cloacae, and Serratia marcescens are the species most often involved in human infections.

Clinical Findings Urinary tract infections and pneumonia are the usual clinical entities associated with these three bacteria, but bacteremia and secondary spread to other areas such as the meninges occur.

Diseases Members of the genus Bacteroidex are the most common cause of serious anaerobic infections, eg. sepsis, peritonitis, and abscesses.

Clinical Findings The B fragilis group of organisms is most frequently associated with intraabdominal infections, either peritonitis or localized abscesses.

Oral, pharyngeal, and pulmonary abscesses are more commonly caused by P melaninogenica, a member of the normal oral flora, but B fragilis is found in about 25% of lung abscesses.

In general, B fragilis causes disease below the diaphragm, whereas P melaninogenica causes disease above the diaphragm.

Laboratory Diagnosis Bacteroides species can be isolated anaerobically on blood agar plates containing kanamycin and vancomycin to inhibit unwanted organisms.

They are identified by biochemical reactions (eg. sugar fermentations) and by production of certain organic acids (eg, formic, acetic, and propionic acids), which are detected by gas chromatography.

Treatment Members of the B fragilis group are resistant to penicillins, first-generation cephalosporins, and aminoglycosides, making them among the most antibiotic-resistant of the anaerobic bacteria.

Penicillin resistance is the result of p-lactamase production.

Beta-lactamase producing strains of P melaninogenica have been isolated from patients.

Prevention Prevention of Bactemiiies and Prevotella infections centers on perioperativc administration of a cephalosporin, frequently cefoxitin, for abdominal or pelvic surgery.

2. What are the differences between the O and H antigens of the Enterobactcriuceae?

5. E coli is one of the two main causes of neonatal meningitis.

6. What is the pathogenesis of E coli-induced diarrhea?

9. Is Salmonella enterocolitis caused by toxins or by invasion of the gut epithelium?

27. Proteus species resemble Salmonella species on TSI agar but can be distinguished by the production of which enzyme?

30. Pus from wound infections caused by Pseudomonas aeruginosa can be blue.