A consolidated analysis of the physiologic and molecular responses induced under acid stress in the legume-symbiont model-soil bacterium Sinorhizobium meliloti.

Abiotic stresses in general and extracellular acidity in particular disturb and limit nitrogen-fixing symbioses between rhizobia and their host legumes. Except for valuable molecular-biological studies on different rhizobia, no consolidated models have been formulated to describe the central physiologic changes that occur in acid-stressed bacteria. We present here an integrated analysis entailing the main cultural, metabolic, and molecular responses of the model bacterium Sinorhizobium meliloti growing under controlled acid stress in a chemostat. A stepwise extracellular acidification of the culture medium had indicated that S. meliloti stopped growing at ca. pH 6.0-6.1. Under such stress the rhizobia increased the O2 consumption per cell by more than 5-fold. This phenotype, together with an increase in the transcripts for several membrane cytochromes, entails a higher aerobic-respiration rate in the acid-stressed rhizobia. Multivariate analysis of global metabolome data served to unequivocally correlate specific-metabolite profiles with the extracellular pH, showing that at low pH the pentose-phosphate pathway exhibited increases in several transcripts, enzymes, and metabolites. Further analyses should be focused on the time course of the observed changes, its associated intracellular signaling, and on the comparison with the changes that operate during the sub lethal acid-adaptive response (ATR) in rhizobia.

Development of new positive-selection RIVET tools: detection of induced promoters by the excision-based transcriptional activation of an aacCI (GmR)-gfp fusion.

RIVET (Recombination Based in vivo Expression Technology) is a powerful genetic tool originally conceived for the identification of genes induced in complex biological niches where conventional transcriptomics is difficult to use. With a broader application, genetic recombination-based technologies have also been used, in combination with regulatory proteins and specific transcriptional regulators, for the development of highly sensitive biosensor systems. RIVET systems generally comprise two modules: a promoter-trap cassette generating genomic transcriptional fusions to the tnpR gene encoding the Tn-γδ TnpR resolvase, and a reporter cassette carrying res-flanked selection markers that are excised upon expression of tnpR to produce an irreversible, inheritable phenotypic change. We report here the construction and validation of a new set of positive-selection RIVET systems that, upon induction of the promoter-trap module, generate the transcriptional activation of an antibiotic-resistant and a green-fluorescent phenotype. Two classes of promoter-trap tools were constructed to generate transcriptional fusions to tnpR: one based on the use of a narrow-host-range plasmid (pRIVET-I), integrative in several Gram-negative bacteria, and the other based on the use of a broad-host-range plasmid (pRIVET-R). The system was evaluated in the model soil bacterium Sinorhizobium meliloti, where a clear-cut phenotypic transition from Nm(R)-Gm(S)-GFP(-) to Nm(S)-Gm(R)-GFP(+) occurred upon expression of tnpR. A S. meliloti integrative RIVET library was constructed in pRIVET-I and, as expected, changes in the extracellular conditions (e.g., salt stress) triggered a significant increase in the appearance of Gm(R)-GFP(+) (excised) clones. The sacB-independent positive-selection RIVET systems here described provide suitable basic tools both for the construction of new recombination-based biosensors and for the search of bacterial markers induced when microorganisms colonize and invade complex environments and eukaryotic hosts.

[Migration to Mexico City: a multifaceted process]. / La migracion a la ciudad de Mexico: un proceso multifacetico.

"In this paper, the author analyzes the evolution of migration to Mexico City based on information generated by the National Survey on Migration to Urban Areas.... The findings support the hypothesis that there has been a decrease in the intensity of in-migration towards capital. The rural origin of the migrants prevails; the majority of these are young people coming from the states closest to the city, with low levels of schooling. An increase is observed in out-migration from Mexico City to other major cities in the country...." (SUMMARY IN ENG)

[Metropolitan areas in Mexico, 1980]. / Zonas metropolitanas en Mexico, 1980.

PIP: Urban trends in Mexico from 1960 to 1980 are analyzed using multivariate analysis techniques. The authors note that the total number of metropolitan regions has increased from 12 to 26 during this period. Differences between the stage in urbanization reached by the Mexico City region and other urban centers in the country are noted. In Mexico City, the authors observe a decline in the population of the central city region coupled with rapid growth in the surrounding municipalities. (SUMMARY IN ENG)^ieng

Estudios de educacion para la salud en el ambito escolar. I. Ausentismo en 1,010 escolares y sus causas.Estudio en dos escuelas primarias del Distrito Federal / Health education studies in schools. I. Absenteeism in 1,010 schoolchildren and motives.Study in two elementary schools of the Distrito Federal

Se llevo al cabo un estudio sobre las razones de ausentismo en 1,010 escolares de dos escuelas primarias del Distrito Federal. Las enfermedades respiratorias superiores, las gastrointestinales y las exantematicas fueron las causas principales de ausentismo por enfermedad. Se analizan la utilizacion de los recursos de salud y las razones de su no utilizacion; asi como las causas de ausentismo no relacionadas con enfermedad. Se deduce que en estudio y tipo de poblacion el ausentismo no es un buen indicador de enfermedad