Growth and Neurodevelopmental Outcomes of Early, High-Dose Parenteral Amino Acid Intake in Very Low Birth Weight Infants: A Randomized Controlled Trial.

BACKGROUND: Administration of high-dose parenteral amino acids (AAs) to premature infants within hours of delivery is currently recommended. This study compared the effect of lower and higher AA administration starting close to birth on short-term growth and neurodevelopmental outcomes at 18-24 months corrected gestational age (CGA). METHODS: Infants <1250 g birth weight (n = 168) were randomly assigned in a blinded fashion to receive parenteral nutrition providing 1-2 g/kg/d AA and advancing daily by 0.5 g/kg/d to a goal of 4 g/kg/d (standard AA) or 3-4 g/kg/d and advancing to 4 g/kg/d by day 1. The primary outcome was neurodevelopmental outcomes measured by the Bayley Scales of Infant and Toddler Development, Third Edition at 18-24 months CGA. Secondary outcomes were growth parameters at 36 weeks CGA among infants surviving to hospital discharge, serum bicarbonate, serum urea nitrogen, creatinine, AA profiles in the first week of life, and incidence of major morbidities and mortality. RESULTS: No differences in neurodevelopmental outcome were detected between the high and low AA groups. Infants in the high AA group had significantly lower mean weight, length, and head circumference percentiles than those in the standard AA group at 36 weeks CGA and at hospital discharge. These differences did not persist after controlling for birth growth parameters, except for head circumference. Infants in the high AA group had higher mean serum urea nitrogen than the standard group on each day throughout the first week. CONCLUSION: Current recommendations for high-dose AA starting at birth are not associated with improved growth or neurodevelopmental outcomes.

Protein intake and neurodevelopmental outcomes.

There is a compelling body of literature that suggests that the provision of an inadequate amount of protein to preterm infants in the neonatal period has detrimental effects on the developing brain with the potential to result in long-term, neurodevelopmental sequelae. Although a great deal of indirect evidence implies that the provision of adequate amounts of protein may be associated with better neurodevelopmental outcomes, there remains a paucity of direct evidence that would allow us to draw any final conclusions.

Adult talk in the NICU with preterm infants and developmental outcomes.

OBJECTIVE: The goal of this study was to test the association of mean adult word counts at 32 and 36 weeks' postmenstrual age in the NICU with Bayley Scales of Infant and Toddler Development, 3rd Edition (Bayley-III), cognitive and language scores. It was hypothesized that preterm infants exposed to higher word counts would have higher cognitive and language scores at 7 and 18 months' corrected age. METHODS: This prospective cohort study included 36 preterm infants with a birth weight ≤1250 g. Sixteen-hour recordings were made in the NICU by using a digital language processor at 32 and 36 weeks' postmenstrual age. Regression analyses were performed on adult word count per hour, with Bayley-III measures correcting for birth weight. RESULTS: Adult word counts in the NICU were positively correlated with 7- and 18-month Bayley-III scores. For the 32-week recording, in regression analyses adjusting for birth weight, adult word count per hour independently accounted for 12% of the variance in language composite scores (P = .04) and 20% of the variance in expressive communication scores at 18 months (P = .008). For the 36-week recording, adult word count per hour independently accounted for 26% of the variance in cognitive composite scores at 7 months (P = .0049). CONCLUSIONS: Increased amount of parent talk with preterm infants in the NICU was associated with higher 7- and 18-month corrected age Bayley-III language and cognitive scores. These findings offer an opportunity for language intervention starting in the NICU.

Cerebral palsy and growth failure at 6 to 7 years.

OBJECTIVE: To evaluate the association between severity of cerebral palsy (CP) and growth to 6 to 7 years of age among children with moderate to severe (Mod/Sev) hypoxic ischemic encephalopathy (HIE). It was hypothesized that children with Mod/Sev CP would have poorer growth, lower cognitive scores, and increased rehospitalization rates compared with children with no CP (No CP). METHODS: Among 115 of 122 surviving children followed in the hypothermia trial for neonatal HIE, growth parameters and neurodevelopmental status at 18 to 22 months and 6 to 7 years were available. Group comparisons (Mod/Sev CP and No CP) with unadjusted and adjusted analyses for growth <10th percentile and z scores by using Fisher's exact tests and regression modeling were conducted. RESULTS: Children with Mod/Sev CP had high rates of slow growth and cognitive and motor impairment and rehospitalizations at 18 to 22 months and 6 to 7 years. At 6 to 7 years of age, children with Mod/Sev CP had increased rates of growth parameters <10th percentile compared with those with No CP (weight, 57% vs 3%; height, 70% vs 2%; and head circumference, 82% vs 13%; P < .0001). Increasing severity of slow growth was associated with increasing age (P < .04 for weight, P < .001 for length, and P < .0001 for head circumference). Gastrostomy feeds were associated with better growth. CONCLUSIONS: Term children with HIE who develop Mod/Sev CP have high and increasing rates of growth <10th percentile by 6 to 7 years of age. These findings support the need for close medical and nutrition management of children with HIE who develop CP.

Elevated temperature and 6- to 7-year outcome of neonatal encephalopathy.

OBJECTIVE: A study was undertaken to determine whether higher temperature after hypoxia-ischemia is associated with death or intelligence quotient (IQ)<70 at 6 to 7 years among infants treated with intensive care without hypothermia. METHODS: Control infants (noncooled, n=106) of the National Institute of Child Health and Human Development Neonatal Research Network hypothermia trial had serial esophageal and skin temperatures over 72 hours. Each infant's temperature was ranked to derive an average of the upper and lower quartile, and median of each site. Temperatures were used in logistic regressions to determine adjusted associations with death or IQ<70 at 6 to 7 years. Secondary outcomes were death, IQ<70, and moderate/severe cerebral palsy (CP). IQ and motor function were assessed with Wechsler Scales for Children and Gross Motor Function Classification System. Results are odds ratio (OR; per degree Celsius increment within the quartile or median) and 95% confidence interval (CI). RESULTS: Primary outcome was available for 89 infants. At 6 to 7 years, death or IQ<70 occurred in 54 infants (37 deaths, 17 survivors with IQ<70) and moderate/severe CP in 15 infants. Death or IQ<70 was associated with the upper quartile average of esophageal (OR=7.3, 95% CI=2.0-26.3) and skin temperature (OR=3.5, 95% CI=1.2-10.4). CP was associated with the upper quartile average of esophageal (OR=12.5, 95% CI=1.02-155) and skin temperature (OR=10.3, 95% CI=1.3-80.2). INTERPRETATION: Among noncooled infants of a randomized trial, elevated temperatures during the first postnatal days are associated with increased odds of a worse outcome at 6 to 7 years.

Screening for autism spectrum disorders in extremely preterm infants.

BACKGROUND: Extremely preterm (EP) infants screen positive for autism spectrum disorders (ASD) at high rates. However, it is not clear whether this is because of high rates of ASD in EPs or to high rates of false-positive screens for ASD in children with a high rate of underlying neurodevelopmental impairments. Combining a parent questionnaire designed to distinguish developmental delay from ASD with direct observation of infant behavior may more accurately screen for ASD in EPs. OBJECTIVES: To determine rates of positive screen for ASD at 18 to 22 months(m) in EPs using 3 screens; to determine factors associated with a positive screen. METHODS: Five hundred fifty-four infants born <27 weeks were screened at 18 to 22 m using the Pervasive Developmental Disorders Screening test, second edition Stage 2, and the response to name and response to joint attention items from the Autism Diagnostic Observation Schedule. Infants with severe cerebral palsy, deafness, and blindness were excluded. Associations between positive screen and neonatal/ infant characteristics were determined. RESULTS: Of 554 infants, 113 (20%) had ≥ 1 positive screen. 10% had a positive Pervasive Developmental Disorders Screening test, second edition, 6% response to name, 9% response to joint attention; in only 1 % all 3 screens were positive. Positive screen was associated with male gender, more hospital days, white race, lower maternal education, abnormal behavioral scores, and cognitive/ language delay. CONCLUSIONS: The use of 3 screens for ASD in EPs results in higher screen positive rates than use of 1 screen alone. Diagnostic confirmation is needed before true rates of ASD in EPs are known.

Are outcomes of extremely preterm infants improving? Impact of Bayley assessment on outcomes.

OBJECTIVES: To compare 18- to 22-month cognitive scores and neurodevelopmental impairment (NDI) in 2 time periods using the National Institute of Child Health and Human Development's Neonatal Research Network assessment of extremely low birth weight infants with the Bayley Scales of Infant Development, Second Edition (Bayley II) in 2006-2007 (period 1) and using the Bayley Scales of Infant and Toddler Development, Third Edition (Bayley III), with separate cognitive and language scores, in 2008-2011 (period 2). STUDY DESIGN: Scores were compared with bivariate analysis, and regression analyses were run to identify differences in NDI rates. RESULTS: Mean Bayley III cognitive scores were 11 points higher than mean Bayley II cognitive scores. The NDI rate was reduced by 70% (from 43% in period 1 to 13% in period 2; P < .0001). Multivariate analyses revealed that Bayley III contributed to a decreased risk of NDI by 5 definitions: cognitive score <70 and <85, cognitive or language score <70; cognitive or motor score <70, and cognitive, language, or motor score <70 (P < .001). CONCLUSION: Whether the Bayley III is overestimating cognitive performance or whether it is a more valid assessment of emerging cognitive skills than the Bayley II is uncertain. Because the Bayley III identifies significantly fewer children with disability, it is recommended that all extremely low birth weight infants be offered early intervention services at the time of discharge from the neonatal intensive care unit, and that Bayley scores be interpreted with caution.

Factors associated with rehospitalizations of very low birthweight infants: impact of a transition home support and education program.

OBJECTIVE: To determine the effects of a transition-home education and support program, BPD, and health insurance type on VLBW infant rehospitalizations at 3 and 7 months corrected age. It was hypothesized that the transition-home program would be associated with decreased rehospitalizations between Phase 1 and 2, and public health insurance and BPD would be associated with increased rehospitalizations. METHODS: 274 infants with birth weight <1500 g were enrolled in two successive years of a transition-home program (Phase 1-start-up) and (Phase 2-full implementation) and followed to 7 months CA. RESULTS: The Phase 2 rehospitalization rates were lower but not statistically significant at both 3 months (20% and 15%; p=0.246), and 7 months (24% and 17%; p=0.171). Infants with public insurance had twice as many rehospitalizations by 3 months (28% versus 11%; p=0.018) in Phase 1. In regression analyses the intervention effects did not achieve significance for the cohort at 3 months (OR=0.63; CI=0.33 to 1.20) or 7 months (OR=0.61; CI=0.33 to 1.13). BPD and public insurance did not reach significance in the models whereas siblings were significantly associated with increased odds of rehospitalization. In subgroup analyses for infants on pubic health insurance the intervention significantly decreased the odds of rehospitalization between Phase 1 and 2(OR=0.43; CI=0.19 to 0.96) at 3 months. CONCLUSIONS: Our findings suggest that a transition-home program may be beneficial to reduce the rehospitalization rate for VLBW infants, and infants on public insurance may derive greater benefit.

Importance of parent talk on the development of preterm infant vocalizations.

OBJECTIVE: To determine the sound environment of preterm infants cared for in the NICU and to test the hypothesis that infants exposed to more adult language will make more vocalizations. METHODS: This was a prospective cohort study of 36 infants who had a birth weight of ≤1250 g. Sixteen-hour recordings of the infant sound environment were made in the NICU from a digital language processor at 32 and 36 weeks' postmenstrual age. Adult word counts, infant vocalizations, and conversational turns were analyzed. RESULTS: Infant vocalizations are present as early as 32 weeks. Both adult word counts per hour and infant vocalizations per hour increase significantly between 32 and 36 weeks. Infant exposure to language as a percentage of time was small but increased significantly. When a parent was present, infants had significantly more conversational turns per hour than when a parent was not present at both 32 and 36 weeks (P < .0001). CONCLUSIONS: Preterm infants begin to make vocalizations at least 8 weeks before their projected due date and significantly increase their number of vocalizations over time. Although infant exposure to language increased over time, adult language accounted for only a small percentage of the sounds to which an infant is exposed in the NICU. Exposure to parental talk was a significantly stronger predictor of infant vocalizations at 32 weeks and conversational turns at 32 and 36 weeks than language from other adults. These findings highlight the powerful impact that parent talk has on the appearance and increment of vocalizations in preterm infants in the NICU.

Impact of very low birth weight infants on the family at 3 months corrected age.

BACKGROUND: Very low birth weight infants (VLBW, <1500 g) have increased impact on families compared to term infants. However, there is limited research examining this impact in the first months post-discharge. AIM: To determine maternal, neonatal, and infant characteristics associated with greater impact on the family at 3 months corrected age in VLBW infants. It was hypothesized that social/environmental and medical risk factors would be associated with higher impact. STUDY DESIGN: Maternal, neonatal, and infant data were collected prospectively. Parents completed the Impact on Family, Family Support, and Family Resource Scales. Associations between characteristics and impact scores were analyzed by t-test and Pearson's correlation. Regression models for each impact score identified significant risk factors for impact. SUBJECTS: 152 VLBW infants born February 28, 2007 to September 5, 2008 who had a follow-up evaluation at 3 months corrected age. OUTCOME MEASURE: Impact on family. RESULTS: Siblings in the home, neonatal medical risk factors, longer hospitalization, more days on ventilator or oxygen, lower gestational age, lower social support, and poorer family resources were associated with increased impact. Multivariate analyses identified siblings in the home, poorer family resources, lower gestational age, and oxygen requirement at 3 months as the most important predictors of impact. CONCLUSIONS: Social/environmental and medical risk factors contribute to impact on family. Families with identified risk factors should receive support services to assist them in coping with the burden of caring for a VLBW infant.

Special health care needs of infants born at the limits of viability.

OBJECTIVE: The objective of this study was to compare rates of survival and special health care needs (SHCN) from discharge to 18 months' corrected age between infants who were born between 22 and 24 weeks and 25 and 27 weeks and to determine predictors and persistence of SHCN. METHODS: Data were collected on 508 infants who were born between 22 and 27 weeks from January 1, 1998, to December 31, 2002 at Women and Infants Hospital. SHCN was defined as need for home oxygen, medication, monitor, gastrostomy tube, or ventriculoperitoneal shunt. chi(2) was used to compare rates of survival and SHCN between groups. Regression analyses explored predictors of SHCN and their persistence. RESULTS: Survival at 22 to 24 weeks was 53% vs 90% at 25 to 27 weeks. There were no 22-week survivors. Survivors at 23 to 24 weeks were more likely to be discharged on oxygen, a monitor, or medications; remain on oxygen or a monitor or require tube feeds at 18 months; and have a SHCN at any time than survivors who were born at 25 to 27 weeks. The strongest predictor of SHCN at discharge was chronic lung disease and at 18 months was public health insurance. CONCLUSIONS: Rates of SHCN were high for infants who were born at the limits of viability. Although rates decreased with increasing age, 40% had persistent SCHN at 18 months. The association of public health insurance with persistent SHCN indicates a need for comprehensive health care and support services for infants with combined biological and environmental risks.

Neurobehavioral assessment predicts motor outcome in preterm infants.

OBJECTIVE: To determine whether Neonatal Intensive Care Unit Network Neurobehavior Scales (NNNS) at 44 weeks predict motor outcome at 2 years in preterm infants from the Maternal Lifestyles Study (MLS). STUDY DESIGN: Data were collected on all preterm infants (<36 weeks) in the MLS who underwent an NNNS at 44 weeks (n = 395) and neurologic examination at 12 to 36 months or Bayley Psychomotor Development Index (PDI) at 24 months (n = 270). Logistic regression analyzed NNNS summary scores associated with cerebral palsy (CP) or PDI <70, while controlling for birth weight

Neurodevelopmental outcome of the premature infant.

Advances in antenatal medicine and neonatal intensive care have successfully resulted in improved survival rates of preterm infants. These improvements have been most dramatic in infants born extremely low birth weight (ELBW,

First-week protein and energy intakes are associated with 18-month developmental outcomes in extremely low birth weight infants.

OBJECTIVE: We sought to evaluate the association between early protein and energy intake and neurodevelopment and growth of extremely low birth weight (<1000 g) infants. STUDY DESIGN: Daily protein and energy intakes were collected by chart review for the first 4 weeks of life on 148 extremely low birth weight survivors. A total of 124 infants (84%) returned for evaluation at 18 months' corrected age. Bivariate analysis tested correlations between weekly protein or energy intakes and Bayley Mental Development Index, Psychomotor Development Index, or growth at 18 months. Separate regression models evaluated contributions of protein (grams per kilogram per day) and energy intake (kilojoules per kilogram per day) to the Mental Development Index, Psychomotor Development Index, and growth, while controlling for known confounders. RESULTS: After adjusting for confounding variables, week 1 energy and protein intakes were each independently associated with the Mental Development Index. During week 1, every 42 kJ (10 kcal)/kg per day were associated with a 4.6-point increase in the Mental Development Index and each gram per kilogram per day in protein intake with an 8.2-point increase in the Mental Development Index; higher protein intake was also associated with lower likelihood of length <10th percentile. CONCLUSIONS: Increased first-week protein and energy intakes are associated with higher Mental Development Index scores and lower likelihood of length growth restrictions at 18 months in extremely low birth weight infants. Emphasis should be placed on providing more optimal protein and energy during this first week.

Function of a chemotaxis-like signal transduction pathway in modulating motility, cell clumping, and cell length in the alphaproteobacterium Azospirillum brasilense.

A chemotaxis signal transduction pathway (hereafter called Che1) has been previously identified in the alphaproteobacterium Azospirillum brasilense. Previous experiments have demonstrated that although mutants lacking CheB and/or CheR homologs from this pathway are defective in chemotaxis, a mutant in which the entire chemotaxis pathway has been mutated displayed a chemotaxis phenotype mostly similar to that of the parent strain, suggesting that the primary function of this Che1 pathway is not the control of motility behavior. Here, we report that mutants carrying defined mutations in the cheA1 (strain AB101) and the cheY1 (strain AB102) genes and a newly constructed mutant lacking the entire operon [Delta(cheA1-cheR1)::Cm] (strain AB103) were defective, but not null, for chemotaxis and aerotaxis and had a minor defect in swimming pattern. We found that mutations in genes of the Che1 pathway affected the cell length of actively growing cells but not their growth rate. Cells of a mutant lacking functional cheB1 and cheR1 genes (strain BS104) were significantly longer than wild-type cells, whereas cells of mutants impaired in the cheA1 or cheY1 genes, as well as a mutant lacking a functional Che1 pathway, were significantly shorter than wild-type cells. Both the modest chemotaxis defects and the observed differences in cell length could be complemented by expressing the wild-type genes from a plasmid. In addition, under conditions of high aeration, cells of mutants lacking functional cheA1 or cheY1 genes or the Che1 operon formed clumps due to cell-to-cell aggregation, whereas the mutant lacking functional CheB1 and CheR1 (BS104) clumped poorly, if at all. Further analysis suggested that the nature of the exopolysaccharide produced, rather than the amount, may be involved in this behavior. Interestingly, mutants that displayed clumping behavior (lacking cheA1 or cheY1 genes or the Che1 operon) also flocculated earlier and quantitatively more than the wild-type cells, whereas the mutant lacking both CheB1 and CheR1 was delayed in flocculation. We propose that the Che1 chemotaxis-like pathway modulates the cell length as well as clumping behavior, suggesting a link between these two processes. Our data are consistent with a model in which the function of the Che1 pathway in regulating these cellular functions directly affects flocculation, a cellular differentiation process initiated under conditions of nutritional imbalance.

NEURODEVELOPMENTAL IMPAIRMENT: PREDICTORS OF ITS IMPACT ON THE FAMILIES OF EXTREMELY LOW BIRTH WEIGHT INFANTS AT 18 MONTHS.

Effects on a family of a child with chronic illness have been described. The Impact on Family Scale (IOF) was developed to measure these effects. The impact of extremely low birth weight (ELBW) infants with neurodevelopmental impairment on families is unknown. This study determined IOF scores for families of ELBW infants with increasing degree of impairment at 18 months and identified factors that increase vulnerability to impact. A total of 3,849 ELBW infant survivors born at the 16 centers of the National Institute of Child Health and Human Development Neonatal Research Network between January 1993 and February 2001 were assessed at 18 to 22 months. Infants were divided into four groups by degree of impairment. IOF scores were analyzed by impairment group. Multivariate analyses assessed effects of impairment, social/demographic factors, unmet service needs, and resource utilization on the IOF. A total of 1,624 (42.2%) infants had moderate/severe impairment. Increasing severity of impairment was associated with higher IOF scores. Severity of impairment contributed 6% of variance to the IOF scores. Twenty-one percent of variance was contributed by additional medical needs, low socioeconomic status (SES), and lack of social support. Although increasing severity of impairment impacts families of ELBW infants, significantly more impact is contributed by additional medical needs, low SES, and lack of social support.

Role of CheB and CheR in the complex chemotactic and aerotactic pathway of Azospirillum brasilense.

It has previously been reported that the alpha-proteobacterium Azospirillum brasilense undergoes methylation-independent chemotaxis; however, a recent study revealed cheB and cheR genes in this organism. We have constructed cheB, cheR, and cheBR mutants of A. brasilense and determined that the CheB and CheR proteins under study significantly influence chemotaxis and aerotaxis but are not essential for these behaviors to occur. First, we found that although cells lacking CheB, CheR, or both were no longer capable of responding to the addition of most chemoattractants in a temporal gradient assay, they did show a chemotactic response (albeit reduced) in a spatial gradient assay. Second, in comparison to the wild type, cheB and cheR mutants under steady-state conditions exhibited an altered swimming bias, whereas the cheBR mutant and the che operon mutant did not. Third, cheB and cheR mutants were null for aerotaxis, whereas the cheBR mutant showed reduced aerotaxis. In contrast to the swimming bias for the model organism Escherichia coli, the swimming bias in A. brasilense cells was dependent on the carbon source present and cells released methanol upon addition of some attractants and upon removal of other attractants. In comparison to the wild type, the cheB, cheR, and cheBR mutants showed various altered patterns of methanol release upon exposure to attractants. This study reveals a significant difference between the chemotaxis adaptation system of A. brasilense and that of the model organism E. coli and suggests that multiple chemotaxis systems are present and contribute to chemotaxis and aerotaxis in A. brasilense.

Lon protease of the alpha-proteobacterium Agrobacterium tumefaciens is required for normal growth, cellular morphology and full virulence.

The ATP-dependent Lon (La) protease is ubiquitous in nature and regulates a diverse set of physiological responses in bacteria. In this paper a lon mutant of the alpha-proteobacterium Agrobacterium tumefaciens C58 has been characterized. Unlike lon mutants of Escherichia coli, the lon mutant of A. tumefaciens grows very slowly, is not filamentous and exhibits normal resistance to UV irradiation. The mutant retains motility and chemotaxis, produces apparently normal amounts of exopolysacchride, but displays severe defects in cell morphology, with 80 % of the mutant cells appearing Y-shaped. Lon protease of A. tumefaciens shares high homology with its counterparts in E. coli and in Sinorhizobium meliloti, and functionally complements an E. coli lon mutant for defects in morphology and RcsA-mediated regulation of capsular polysaccharide production. Mutations at sites of Lon(At) corresponding to the ATP-binding site and the active site serine of the E. coli Lon protease abolish complementation of phenotypes of the A. tumefaciens and E. coli lon mutants. The nucleotide sequence upstream of A. tumefaciens lon contains an element similar to the consensus sigma(32) heat-shock promoter of E. coli. Northern and Western blot analyses indicated that expression of lon is induced by elevated temperature, albeit to a much lower level than that of groEL. The lon mutant is highly attenuated for virulence, suggesting that Lon may be required for the proper expression, assembly or function of the VirB/D4-mediated T-DNA transfer system.

An energy taxis transducer promotes root colonization by Azospirillum brasilense.

Motility responses triggered by changes in the electron transport system are collectively known as energy taxis. In Azospirillum brasilense, energy taxis was shown to be the principal form of locomotor control. In the present study, we have identified a novel chemoreceptor-like protein, named Tlp1, which serves as an energy taxis transducer. The Tlp1 protein is predicted to have an N-terminal periplasmic region and a cytoplasmic C-terminal signaling module homologous to those of other chemoreceptors. The predicted periplasmic region of Tlp1 comprises a conserved domain that is found in two types of microbial sensory receptors: chemotaxis transducers and histidine kinases. However, the function of this domain is currently unknown. We characterized the behavior of a tlp1 mutant by a series of spatial and temporal gradient assays. The tlp1 mutant is deficient in (i) chemotaxis to several rapidly oxidizable substrates, (ii) taxis to terminal electron acceptors (oxygen and nitrate), and (iii) redox taxis. Taken together, the data strongly suggest that Tlp1 mediates energy taxis in A. brasilense. Using qualitative and quantitative assays, we have also demonstrated that the tlp1 mutant is impaired in colonization of plant roots. This finding supports the hypothesis that energy taxis and therefore bacterial metabolism might be key factors in determining host specificity in Azospirillum-grass associations.