How microbiological tests reflect bacterial pathogenesis and host adaptation.

Historically, clinical microbiological laboratories have often relied on isolation of pure cultures and phenotypic testing to identify microorganisms. These clinical tests are often based on specific biochemical reactions, growth characteristics, colony morphology, and other physiological aspects. The features used for identification in clinical laboratories are highly conserved and specific for a given group of microbes. We speculate that these features might be the result of evolutionary selection and thus may reflect aspects of the life cycle of the organism and pathogenesis. Indeed, several of the metabolic pathways targeted by diagnostic tests in some cases may represent mechanisms for host colonization or pathogenesis. Examples include, but are not restricted to, Staphylococcus aureus, Pseudomonas aeruginosa, Klebsiella pneumoniae, Salmonella enterica, Shigella spp., and enteroinvasive Escherichia coli (EIEC). Here, we provide an overview of how some common tests reflect molecular mechanisms of bacterial pathogenesis.

Subtyping of Escherichia coli O157:H7 strains isolated from human infections and healthy cattle in Argentina.

Shiga toxin-producing Escherichia coli (STEC) cause nonbloody (NBD) and bloody diarrhea (BD), and hemolytic uremic syndrome (HUS). Cattle have been described as their main reservoir. STEC O157:H7 is recognized as the predominant serotype in clinical infections, but much less is known about the dominant subtypes in humans and animals or their genetic relatedness. The aims of this study were to compare the STEC O157 subtypes found in sporadic human infections with those in the bovine reservoir using stx-genotyping, phage typing, and XbaI-pulsed-field gel electrophoresis (PFGE), and correlate the subtypes with the severity of clinical manifestations. The 280 STEC O157:H7 strains collected included in this study were isolated from HUS (n=122), BD (n=69), and NBD (n=30) cases, and healthy carriers (n=5), and from bovines (n=54) in the abattoirs. The stx-genotyping showed that stx2/stx(2c(vh-a)) was predominant in human (76.1%) and in bovine strains (55.5%), whereas the second more important genotype was stx2 (20.8%) in human and stx(2c(vh-a)) (16.7%) in cattle strains. In human strains, PT4 (37.6%), PT49 (24.3%), and PT2 (18.6%) were the most frequent PTs (80.5%). In bovine isolates, PT2 (26%), PT39 (16.7%), and PT4 and PT49 (11.1% each) were predominant. By XbaI-PFGE, all 280 strains yielded 148 patterns with 75% similarity, and 169 strains were grouped in 37 clusters. Identical PT-PFGE-stx profile combinations were detected in strains of both origins: PT4-AREXH01.0011-stx2/stx(2c(vh-a)) (12 humans and one bovine), PT4-AREXH01.0543-stx2/stx(2c(vh-a)) (one human and four bovines), PT2-AREXH01.0076-stx2/stx(2c(vh-a)) (one human and four bovines), PT49-AREXH01.0175-stx2/stx(2c(vh-a)) (seven humans and one bovine), and PT49-AREXH01.0022-stx2/stx(2c(vh-a)) (seven humans and one bovine). No correlation was found among the stx-genotypes, the phage type, and the clinical symptoms.

Current aspects of Shigella pathogenesis.

Bacillary dysentery (shigellosis) is a severe human disease caused by Shigellae. In recent years, a large amount of information has been generated regarding the host, pathogen and environmental factors that impact the pathogenesis of shigellosis at the cellular and molecular level. This review summarizes what is currently known about Shigella, detailing those factors that contribute to pathogenesis and examining the current progress in the development of a vaccine.

[Molecular biology in the pathogenesis of Shigella sp. and enteroinvasive Escherichia coli]. / Biología molecular en la patogenia de Shigella sp. y Escherichia coli enteroinvasiva.

Shigella sp and Escherichia coli (EIEC) are casual agents of bacillary dysentery, mainly in developing countries. Shigella and EIEC share biochemical, antigenic and genetic properties and probably they have the same mechanism of pathogenicity. Both species harbor a 120-140 megadalton plasmid, which is associated to the virulence and whose expression is regulated by chromosomal genes. Shigella sp and EIEC invade colonic epithelium and present virulence auxiliary factors, such as mucinases, superoxide dismutase and aerobactine production. On the other hand, cytotoxin production contributes to the illness' severity. The first step in invasion of the colonic mucosa is epithelium adherence, followed by endocytosis, lysis of the phagocytic vacuole, intracellular multiplication, intra-intercellular spread and killing of the host cell. Identification of these invasive organisms is carried out with the Sereny test, chicken embryo lethality and invasion to culture cells assays, DNA probe hibridization, polimerase chain reaction, ELISA, Congo red binding, and biochemical and serological tests.

Asymptomatic Shigella infections in a cohort of Mexican children younger than two years of age.

The proportion of Shigella infections that occur asymptomatically in young children has not been established. A community-based cohort study of 367 infants was followed prospectively by weekly home visits from January, 1990, through December, 1991. Stool samples were collected weekly and when diarrhea occurred and were tested for Shigella and other enteropathogens. There were 2925 child months of observation and 65 episodes of Shigella infection. There were 3.1 episodes/100 child months during the warm season (May through September) and 0.97 episode/100 child months during the cold season. Shigella infections were rare during the first 6 months of life but increased with age (P < 0.0001). Overall 55% of detected infections were asymptomatic. The proportion of infections that were asymptomatic increased as age increased (P < 0.01). Symptom status was not significantly associated with Shigella species or season. All isolates from symptomatic and asymptomatic children had the 120- to 140-megadalton virulence plasmid. We conclude that infections with virulent strains of Shigella are commonly asymptomatic in Mexican children during the first 2 years of life.

PIP: During January 1990-December 1991, each week, field workers visited the home of 367 children aged 0-24 months from a periurban area southwest of Mexico City (San Pedro Martir and San Andres Totoltepec, Tlalpan) and collected stool specimens from them to determine whether Shigella infections are often asymptomatic. The crude incidence rate of diarrhea, regardless of etiology, was 29 episodes/100 child months during the warmer and rainy months (May-September), while it was 21 episodes/100 child months for the rest of the year (October-April) (relative risk [RR] =1.38). 53 of all children (l4%) had 65 Shigella infections. The overall monthly incidence of symptomatic and asymptomatic Shigella infection was higher during May-September than October-April (3.13 vs. 0.97 episodes/100 children; RR = 3.22). 55% of all Shigella infections (36) were asymptomatic. 32% developed secretory-type diarrhea and 13% had blood present in the stool. The incidence of Shigella infections grew as did the age (0.4-8.2 episodes/100 child months for 0-6 month olds to 18-24 month olds; p 0.0001). The proportion of asymptomatic Shigella infections also increased with age (33% for 0-6 month olds, 40% for 7-12 month olds, 46% for 13-18 month olds, and 78% for 18-24 month olds; p 0.01). Shigella sonnei, S. flexneri, and S. boydii were the only species detected. The 120-140 megadalton virulence plasmid was present in all isolates from asymptomatic and symptomatic children. Mixed infections were rather common in both asymptomatic (47%) and symptomatic (45%) children. Among infants aged less than 12 year months, breast feeding infants were less likely to be infected with Shigella than nonbreast feeding infants (RR = 2.41). On the other hand, among children aged 12-24 months, nonbreast feeding was associated with a lower risk of Shigella infection (RR = 0.69). These findings show that Shigella infections in Mexican children aged 0-24 months range from asymptomatic infections to secretory diarrhea to bloody diarrhea.

[Shigellosis in children of the IX region of Chile: clinical and epidemiologic aspects and antibiotic sensitivity]. / Shigellosis infantil en la IX region: aspectos clinicos, epidemiologicos y estudio de sensibilidad.

Ninety four children with diarrhea and a positive stool culture for Shigella, hospitalized at the Hospital Regional de Temuco, were studied. Forty six percent of patients were less than two years old. Forty two percent of microorganisms were resistant to Ampicillin, 45% to trimethoprim/sulfamethoxazole, 8% to tetracycline and none to chloramphenicol. Isolated species were Shigella flexneri 83% and Shigella sonnei 17%. Seventy nine percent of patients had fever, 60% dysentery and 21.3% seizures. Ninety two percent of symptomatic family contacts had a positive stool culture for Shigella. Due to the high incidence of resistance to ampicillin and trimethoprim/sulfamethoxazole, these antimicrobials are not recommended as the first choice treatment of Shigellosis in the Ninth region of Chile.

Urgencias gastrointestinales

Urgencias gastrointestinales con énfasis en diagnóstico y tratamiento. Destaca el rol de la arteriografía