Gram Negative Rods 2020.pdf

Gram Negative Rods Dr. Suhad Hadi Mohammed Enterobacteriaceae The Enterobacteriaceae are a large, heterogeneous group of gram-negative rods whose natural habitat is the intestinal tract of humans and animals. The family includes many genera. 63 genera have been defined; however, the clinically...

Gram Negative Rods Dr. Suhad Hadi Mohammed Enterobacteriaceae The Enterobacteriaceae are a large, heterogeneous group of gram-negative rods whose natural habitat is the intestinal tract of humans and animals. The family includes many genera. 63 genera have been defined; however, the clinically significant Enterobacteriaceae comprise 20–25 species, and other species are encountered infrequently. Escherichia, Shigella, Salmonella, Enterobacter, Klebsiella, Serratia, Proteus, Cirtobacter, Yersinia, and others. Some enteric organisms, such as Escherichia coli, are part of the normal microbiota and incidentally cause disease, but others, the salmonellae and shigellae, and Yersinia are regularly pathogenic for humans.  Members of the family Enterobacteriaceae have the following characteristics: 1. They are gram-negative rods. 2. Either motile with peritrichous flagella or nonmotile. 3. Grow well on MacConkey agar. 4. are facultative anaerobes. 5. Ferment rather than oxidize glucose, often with gas production. 6. Catalase positive. 7. Oxidase negative. 8. Reduce nitrate to nitrite. Antigenic Structure Enterobacteriaceae have a complex antigenic structure. 1. More than 150 different heat stable somatic O (lipopolysaccharide) antigens. 2. More than 100 heat-labile K (capsular) antigens. In Salmonella Typhi, the capsular antigens are called Vi antigens. 3. More than 50 H (flagellar) antigens. O antigens are the most external part of the cell wall lipopolysaccharide and consist of repeating units of polysaccharide. Antibodies to O antigens are predominantly IgM. H antigens are located on flagella and are denatured or removed by heat or alcohol. They are preserved by treating motile bacterial variants with formalin. Such H antigens agglutinate with anti-H antibodies, mainly IgG. A. E COLI 1. Urinary tract infection E coli is the most common cause, 90% of first urinary tract infections in young women. The signs and symptoms include urinary frequency, dysuria, hematuria, and pyuria. Nephropathogenic E coli typically produce a hemolysin. 2. E coli-associated diarrheal diseases E coli that can cause diarrhea are classified by the characteristics of their virulence properties, and each group causes disease by a different mechanism. Enteropathogenic E coli (EPEC)  Diarrhea in infants, especially in developing countries.  Adhere to the mucosal cells of the small bowel. Chromosomally mediated factors promote tight adherence.  watery diarrhea usually self-limited but can be chronic. Enterotoxigenic E coli (ETEC)  Cause of “traveler’s diarrhea” and diarrhea in infants in developing countries.  colonization factors specific promote adherence to epithelial cells of the small bowel.  Some strains produce a heat-labile exotoxin (LT):  AB toxin.  Under the control of a plasmid.  cause prolonged hyper secretion of water and chlorides and inhibits the re-absorption of sodium.  LT is antigenic and cross-reacts with the enterotoxin of Vibrio cholerae.  Some strains produce the heat-stable enterotoxin STa:  under the control of a heterogeneous group of plasmids.  STa stimulates fluid secretion.  Many STa-positive strains also produce LT. Enterohemorrhagic E coli (EHEC)  Produces verotoxin, named for its cytotoxic effect on Vero cells.  Associated with hemorrhagic colitis, a severe form of diarrhea, and with hemolytic uremic syndrome, a disease resulting in acute renal failure.  Verotoxin has many properties that are similar to the Shiga toxin produced by some strains of Shigella dysenteriae type 1  Serotypes O157:H7 (does not use sorbitol, unlike most other E coli, and is negative on sorbitol MacConkey agar -- sorbitol is used instead of lactose). Enteroinvasive E coli (EIEC)  Disease very similar to shigellosis (invading intestinal mucosal epithelial cells)  Disease occurs most commonly in children in developing countries and in travelers to these countries.  Like shigella, EIEC strains are nonlactose or late lactose fermenters and are nonmotile. Enteroaggregative E coli (EAEC)  Cause: acute and chronic diarrhea in persons in developing countries and food-borne illnesses in industrialized countries.  Poses characteristic pattern of adherence to human cells.  Produce ST-like toxin and a hemolysin. 3. Sepsis—When normal host defenses are inadequate, E coli may reach the bloodstream and cause sepsis. 4. Meningitis—E coli and group B streptococci are the leading causes of meningitis in infants. B. Klebsiella—K pneumoniae is present in the respiratory tract and feces of about 5% of normal individuals. It causes a small proportion (about 1%) of bacterial pneumonias. It occasionally produces urinary tract infection and bacteremia with focal lesions in debilitated patients. C. Enterobacter aerogenes—This organism has small capsules, may be found free-living as well as in the intestinal tract, and causes UTI and sepsis. D. Serratia—S marcescens is a common opportunistic pathogen in hospitalized patients. E. Proteus—P mirabilis causes UTI and occasionally other infections. Proteus species produce urease, resulting in rapid hydrolysis of urea with liberation of ammonia. Thus, the urine becomes alkaline, promoting stone formation. The rapid motility of Proteus may contribute to its invasion of the urinary tract. F. Providencia—Providencia species are members of the normal intestinal flora. All cause UTI and occasionally other infections and are often resistant to antimicrobial therapy. G. Citrobacter—cause UTI and sepsis. H. Shigellae With the exception of Shigella sonnei, they do not ferment lactose. LNF Shigellae form acid from carbohydrates but rarely produce gas. They may also be divided into those that ferment mannitol and those that do not. There are more than 40 serotypes (depending on polysaccharide O antigen). Pathogenesis & Pathology incubation period (1–2 days) Shigella infections are almost always limited to the gastrointestinal tract; bloodstream invasion is quite rare. the infective dose is 103 organisms (whereas it usually is 105–108 for salmonellae and vibrios). The essential pathologic process is 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. Toxins A. ENDOTOXIN Upon autolysis, all shigellae release their toxic lipopolysaccharide. This endotoxin probably contributes to the irritation of the bowel wall. B. SHIGELLA DYSENTERIAE EXOTOXIN S dysenteriae type 1 (Shiga bacillus) produces a heat-labile exotoxin that affects both the gut and the central nervous system. I. Salmonellae 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. The classification of salmonellae is complex. 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. The infective dose is 105–108 salmonellae (but perhaps as few as 103 Salmonella Typhi organisms). THE “ENTERIC FEVERS” (TYPHOID FEVER), systemic infection  Salmonella Typhi  Incubation period of 10–14 days  The ingested salmonellae reach the small intestine, from which they enter the lymphatics and then the bloodstream.  Salmonella carried by the blood to many organs, including the intestine.  The organisms multiply in intestinal lymphoid tissue and are excreted in stools.  Blood cultures are often positive in the first week of the disease. Bone marrow cultures may be useful. Urine cultures may be positive after the second week. Stools yield positive results from the second or third week. Gastroenteritis (Salmonellosis)  Salmonella Typhimurium and Salmonella Enteritidis are prominent cause  I.P. Eight to 48 hours  Cause profuse diarrhea, with few leukocytes in the stools.  Inflammatory lesions of the small and large intestine are present. Bacteremia is rare (2–4%) except in immunodeficient persons. Blood cultures are usually negative, but stool cultures are positive for salmonellae and may remain positive for several weeks after clinical recovery. VIBRIO Members of the genus Vibrio are short, curved, rod-shaped organisms. They are rapidly motile by means of a single polar flagellum. O and H antigens are both present, but only O antigens are useful in distinguishing strains of vibrios that cause epidemics. Vibrios are facultative anaerobes. The growth of many Vibrio strains either requires or is stimulated by NaCl. Vibrio grow in synthetic media with glucose as carbon and energy source. Vibrio tolerate alkaline and sensitive to acid. Pathogenic vibrios include: 1) Vibrio cholerae, serogroup O1 strains that are associated with epidemic cholera. This serotype is further differentiated serologically according to their subsidiary O-Antigen into INABA, OGAWA, HIKOJIMA (the names denoting their origin 2) non-O1 V. cholerae and related strains that cause sporadic cases of cholera-like and other illnesses 3) Vibrio parahaemolyticus and other halophilic vibrios, which cause gastroenteritis and extraintestinal infections. Epidemiology V. cholerae is transmitted to humans by contaminated water and food. In the aquatic environment. The infectious dose varies depending on the pH of the stomach, in healthy volunteers 108 bacteria produce infection. The small intestine is the primary site of infection. And V. cholera colonizes the epithelium without invasion or apparent damage. cholera is characterized by massive loss of fluid and electrolytes from the body (“rice-water” stools). Untreated, death from severe dehydration causing hypo- volemic shock may occur in hours to days, and the death rate may exceed 50 percent. Virulance factor Cholera toxin (CT) composed of AB subunits. Once inside the cell (A)subunit causes changes in the regulation of cell genes and the result flow of ions and water is reversed. Laboratory identification V. cholerae grows on standard media such as blood and MacConkey agars. Thiosulfate-citrate-bile salts–sucrose (TCBS) medium can enhance isolation and it required transport media (Carry Blair media). The organism is oxidase positive.  PSEUDOMONAS (P. aeruginosa) Widely distributed in nature (found in soil, water, plants, and animals). May colonize healthy humans without causing disease, it is also a significant opportunistic pathogen and a major cause of nosocomial (hospital-acquired) infections. Regularly a cause of nosocomial pneumonia, nosocomial urinary tract infections, surgical site infections, infections of severe burns, and infections of patients undergoing either chemotherapy for neoplastic disease or antibiotic therapy. Motile (it has polar flagella) Aerobic or facultative. Does not ferment carbohydrates but can utilize alternate electron acceptors, such as nitrate, in anaerobic respiration. Nutritional requirements are minimal, and the organism can grow on a wide variety of organic substrates. In fact, P. aeruginosa can even grow in laboratory water baths, hot tubs, intravenous (IV) tubing, and other water-containing vessels. This explains why the organism is responsible for so many nosocomial infections. Pathogenesis Disease begins with attachment to and colonization of host tissue. Pili on the bacteria mediate adherence, and mucoid strains predominate in patients with cystic fibrosis (CF). The mucoid capsule is called alginate which confers resistance to phagocytosis and clearing in the CF lung. Host tissue damage facilitates adherence and colonization. P. aeruginosa produces numerous toxins and extracellular products that promote local invasion and dissemination of the organism (causes both localized and systemic illness) Individuals most at risk include those with impaired immune defenses. Laboratory identification samples plated on a variety of media, both nonselective (blood agar) and moderately selective (MacConkey agar). Identification is based on the results of biochemical and other diagnostic tests. P. aeruginosa typically produces a blue-green pigment called pyocyanin and is oxidase positive. Acinetobacter Gram-negative coccobacilli On primary isolation, they closely resemble the Enterobacteriaceae in growth pattern and colonial morphology but are distinguished by their failure to ferment carbohydrates or reduce nitrates. Pneumonia is the most common infection, followed by urinary tract and soft tissue infections. Nosocomial respiratory infections have been traced to contaminated inhalation therapy equipment, and bacteremia to infected intravenous catheters. Treatment is complicated by frequent resistance to penicillins, cephalosporins, and occasionally aminoglycosides. Epidemiology Acinetobacter baumannii accounts for about 80% of reported infections. Transmission of Acinetobacter infection Acinetobacter poses very little risk to healthy people. However, people who have weakened immune systems, chronic lung disease, or diabetes may be more susceptible to infections with Acinetobacter. Hospitalized patients, especially very ill patients on a ventilator, those with a prolonged hospital stay, those who have open wounds, or any person with invasive devices like urinary catheters are also at greater risk for Acinetobacter infection. Acinetobacter can be spread to susceptible persons by person-to-person contact or contact with contaminated surfaces. Lab Identification Grow well on MacConkey agar (None-lactose-fermenting or partially lactose-fermenting). They are oxidase-negative, catalase-positive, indole-negative, nonmotile, and usually nitrate-negative. Thank you