Case 1 introduction

 Most likely etiology of infection: Enterococcus faecalis.

Clinical diagnosis of urinary tract infections is made by typical clinical symptoms of urgency and/or dysuria followed by a urinalysis and bacterial culture. The presence of white blood cells (positive leukocyte esterase) and bacteria in the urine are indicative of cystitis. The specific etiologic agent can only be determined by culturing the urine in a quantitative manner. The presence of greater than 10⁵ colony-forming units (CFUs) per ml of clean catch urine or 10⁴ CFU/ml of catheterized urine is considered significant for a urinary tract infection. Colonies of Enterococcus appear nonhemolytic or, in rare cases, alpha hemolytic on blood agar and can be specifically identified using a rapid PYR (L-pyrrolidonyl--naphthylamide) test. Conventional, overnight identification includes growth in 6.5% sodium chloride and esculin hydrolysis in the presence of bile. Further identification of Enterococci to the species level is not commonly done in routine clinical laboratories. Although most commercially available identification methods can speciate Enterococci difficulties in accurate speciation occurs without the use of DNA sequence analysis. Most clinically significant Enterococci are either E. faecalis or E. faecium. E. faecium tends to be more resistant to antibiotics particularly ampicillin and vancomycin.

Although Enterococci are intrinsically resistant to low concentrations of beta-lactam antibiotics, such as ampicillin, these agents are still the first choice for uncomplicated enterococcal urinary tract infections in cases in which the affecting strain is not highly resistant. For complicated urinary tract infections or endocarditis, bactericidal therapy is necessary and includes ampicillin or vancomycin plus an aminoglycoside, assuming that the infecting strain is susceptible to ampicillin or vancomycin and high levels of aminoglycosides. An alternative would be vancomycin if it is susceptible; or if resistant, alternative agents such as linezolid or quinupristin/dalfopristin might be appropriate.

Although there is no specific prevention for Enterococci because they are able to survive for extended periods of time on inanimate objects, nosocomial outbreaks have been associated with antibiotic-resistant strains of Enterococci and proper disinfection and infection control measures are necessary to prevent further spread.

COMPREHENSION QUESTIONS

[8.1] Testing of blood culture isolates from a hospitalized patient revealed gram-positive cocci, -lactamase positive, vancomycin-resistant, PYR-positive, and the presence of Lancefield group D antigen. Which of the following is the most likely isolate identification?

[8.2] Which of the following is the most serious condition that can result as complications of enterococcal urinary tract or wound infections?

[8.3] After an abdominal surgery for removal of ovarian cysts, this 56-year-old woman has had low-grade fever for the past two weeks. She has a history of rheumatic fever as a child. Three of the blood cultures grew gram-positive cocci. Which of the following is the most likely etiologic agent?

[8.1] A. All bacteria listed are gram-positive cocci. S. pneumoniae does not have a Lancefield grouping, whereas S. agalactiae has a group B classification. S. bovis is PYR-negative. Thus, only E. faecalis fulfills all laboratory test results in the above question.

[8.2] D. In patients, the most common sites of Enterococci infection are the urinary tract, wounds, biliary tract, and blood. In neonates, Enterococci can cause bacteremia and meningitis. In adults, enterococci may cause endocarditis. Thus, bacteremia and/or endocarditis are rare and very serious complications that can result from enterococcal urinary tract infections. Cellulitis and toxic shock syndrome are typically associated with both Staphylococci and Streptococci, whereas scarlet fever is associated only with Streptococci. Finally, gastroenteritis can be associated with a number of organisms such as Clostridium difficile.

[8.3] D. Streptococcus bovis is among the nonenterococcal group D Streptococci. They are part of the enteric flora and have the ability to cause endocarditis.

A 21-year-old woman presents to the office with a 3-day duration of discomfort with urination and increased urinary frequency. She has noted that her urine has a strong odor as well. She denies fever, abdominal pain, back pain, vaginal discharge, or skin rash. She is sexually active and takes oral contraceptive pills. On examination, she is comfortable appearing and afebrile. She has no costovertebral angle tenderness. Abdominal exam is notable only for suprapubic tenderness. Microscopic examination of the sediment of a centrifuged urine sample reveals 10-15 white blood cells per high power field and numerous bacteria.

 What type of organism would a Gram stain of the urine most likely show?

 What is the most common etiologic agent of this infection?

 What is the most likely reservoir of this infection?

 What is the most likely mechanism by which this organism infects the urinary tract?

 Organism most likely to be seen on Gram stain: Gram-negative rod.

 Most common etiologic agent: Escherichia coli (E. coli).

 Most likely reservoir for the organism: Patient's own gastrointestinal (GI) tract.

 Most likely mechanism of introduction of organism into the urinary tract: Urethral contamination by colonic bacteria followed by ascension of the infection into the bladder.

Summary: A 21-year-old woman has urinary frequency and dysuria. The urinalysis shows numerous white blood cells.

CLINICAL CORRELATION

Introduction

E. coli is the most commonly found aerobic, gram-negative bacilli in the GI tract of humans. E. coli is responsible for over 80 percent of all urinary tract infections (UTIs), along with other clinical diseases including gastroenteritis, sepsis, and neonatal meningitis. The E. coli that causes diarrhea is usually acquired from the environment, whereas most other infections caused by E. coli are acquired endogenously. Much of the diarrhea resulting from E. coli is acquired in developing countries particularly in travelers to these countries. The serotypes that are associated with travelers' diarrhea can be grouped based on their method of pathogenesis: enterotoxigenic, enterohemorrhagic, enteroaggregative, and enteroinvasive strains. These strains produce toxins, which account for their invasiveness as well as decreased absorption in the GI tract. Most of these cause a self limited diarrhea with the exception of enterohemorrhagic E. coli, frequently known as E. coli serotype O157:H7, which is usually acquired from eating poorly cook meat from an infected cow. Complications of infection with this organism include hemolytic uremic syndrome (HUS), which is a triad of hemolytic anemia, thrombocytopenia, and anemia. HUS is a significant cause of acute renal failure in children.

Urinary tract infections caused by E. coli are associated with organisms from the GI tract or vagina ascending up to the bladder. These organisms can colonize the vagina and be introduced into the bladder during instrumentation or sexual intercourse. Those serotypes that produce adhesions, which mediate adherence of the organisms to epithelial cells in the urinary tract are more likely to cause infections. The majority of cases of uncomplicated and complicated pyelonephritis are caused by E. coli, a complication of a UTI, where the organisms continue to ascend from the bladder to the kidney.

The diagnosis of a UTI is made by urinalysis and urine culture. Complications such as pyelonephritis would be indicated by fever and flank pain. On urinalysis, the presence of white blood cells or leukocyte esterase and bacteria are suggestive of a true infection. Definitive diagnosis of the etiology is made by culture of the urine. E. coli is easily grown on most culture media. A quantitative urine culture from a symptomatic patient should demonstrate greater than 10⁵ colony-forming units (CFUs) bacteria/ml urine to be considered significant. E. coli would appear as pink colonies on MacConkey agar indicating fermentation of lactose. A rapid spot indole test would give a preliminary identification of E. coli, which would be confirmed by biochemical analysis.

Treatment of UTIs is based on the affecting organism and its susceptibility to antibiotics. Common antimicrobials chosen include trimethoprim sulfamethoxazole, or a fluoroquinolone. Most E. coli are resistant to ampicillin and penicillin. Recurrent UTIs are quite common, particularly in young women. Prevention can include consumption of large amounts of liquid and attention to totally emptying the bladder during urination. Fluid and electrolyte replacement should be administered to patients with E. coli diarrhea; however, antimicrobial treatment should not be administered. E. coli diarrhea is best prevented by improved hygiene.

[9.1] Escherichia coli can be classified by their characteristic virulence properties and different mechanisms that cause disease. To which group does the verotoxin producing E. coli 0157:H7 serotype belong?

[9.2] Several children are hospitalized with bloody diarrhea and severe hematological abnormalities. A 4-year-old girl dies of kidney failure shortly after admittance. An epidemiological investigation establishes that all of the patients developed symptoms following consumption of hamburgers from the same fast-food restaurant chain. Which of the following organisms is most likely to be responsible for the outbreak?

[9.3] A Gram stain of an isolate from voided urine in a patient with a UTI reveals the presence of pink rods. Further biochemical analysis reveals that these bacteria ferment glucose, reduce nitrates to nitrites and are unable to synthesize the following reaction:

2H⁺ + 2e^- + 1/2 O₂  H₂O

Which of the following characterize the above test results?

[9.4] A 7-year-old child with bloody diarrhea is admitted after lab results indicating anemia and abnormal kidney function. After testing, it is determined that the etiologic agent is an E. coli that is most likely to produce which of the following?

[9.5] Several days after an appendectomy a patient develops a high fever, dangerously low blood pressure, and disseminated intravascular coagulation (DIC). Based on these and other findings, a diagnosis of septicemia as a result of an enteric gram-negative rod is made. Which of the following cytokines is most likely to be responsible for the fever, low blood pressure, and DIC?

[9.1] B. All of the above classes of E. coli cause diarrhea. However, only EHEC produce a verotoxin that has many properties that are similar to Shiga toxin. EHEC has been associated with hemorrhagic colitis, a severe form of diarrhea, and with hemolytic uremic syndrome (HUS). HUS is a disease resulting in acute renal failure, microangiopathic hemolytic anemia, and thrombocytopenia.

[9.2] C. Escherichia coli 0157:H7 strains are classically associated with outbreaks of diarrhea after ingestion of undercooked hamburger at fast-food restaurants. Many cases of hemorrhagic colitis and its associated complications can be prevented by thoroughly cooking ground beef. The other organisms listed can cause gastrointestinal disturbances; however, E. coli is the classic disturbing pathogen in this case. Shigella dysenteriae produces a heat-labile enterotoxin that affects the gut and central nervous system and is a human disease that is transmitted via a fecal-oral route. Salmonella and Campylobacter are associated with poultry and eggs primarily, whereas Vibrio is associated mainly with seafood.

[9.3] A. The biochemical reaction described above is catalyzed by the enzyme oxidase. Thus, E. coli is the only bacteria listed that is a gram-negative rod (pink), ferments glucose, converts nitrates to nitrites, and is oxidase negative.

[9.4] E. The verotoxin produced by E. coli is similar to Shiga toxin, causing bloody diarrhea. Please refer to the discussion for Question 9.1.

[9.5] E. The acute phase response involves the increase in the levels of various plasma proteins (C-reactive protein and mannose-binding proteins) and is part of innate immunity. These proteins are synthesized and secreted by the liver in response to certain cytokines such as IL-1, IL-6, and TNF-alpha (produced after exposure to microorganisms) as nonspecific responses to microorganisms and other forms of tissue injury. Specifically, endotoxin (LPS) from gram-negative bacteria has the ability to activate macrophages that, in turn, synthesize TNF-alpha. TNF-alpha then functions to cause fever and hemorrhagic tissue necrosis (inflammatory reaction/immune response).

A 48-year-old man presents for the evaluation of a 2-month history of upper abdominal pain associated with nausea. It is made worse when he drinks coffee, soda, or alcohol. He has taken multiple over-the-counter antacid medications that provide temporary relief. He admits to a 20-pack-year smoking history and drinking one or two alcoholic beverages a week but denies significant use of nonsteroidal antiinflammatory drugs (NSAIDs). His general physical examination and vital signs are normal. His abdominal examination is notable for epigastric tenderness without the presence of masses, rebound tenderness, or guarding. A rectal examination reveals his stool to be heme positive. A CBC shows a mild hypochromic, microcytic anemia. He is referred to a gastroenterologist for an upper GI endoscopy, which shows diffuse gastritis and a gastric ulcer.

 What organism is most likely to be visualized on histologic evaluation of a gastric biopsy specimen?

 Besides microscopic evaluation, what other clinical test may provide a rapid detection of this organism?

 What two factors facilitate this organism's ability to colonize the stomach?

ANSWERS TO CASE 10: Helicobacter pylori

 Organism on histologic evaluation of a gastric biopsy: The organism likely to be visualized on biopsy specimen is Helicobacter pylori.

 Other clinical test for rapid detection of this organism: The urease test.

 Factors facilitating this organism's ability to colonize the stomach: Blockage of acid production by a bacterial acid-inhibitory protein and neutralization of acid by ammonia produced by urease activity.