Pneumocystis colonisation is common among hospitalised HIV infected patients with non-Pneumocystis pneumonia

BACKGROUND

When Pneumocystis DNA is recovered from respiratory specimens of patients without Pneumocystis pneumonia (PCP), patients are said to be colonised with Pneumocystis, although the significance of this state is unknown. Understanding risk factors for and outcomes of colonisation may provide insights into the life cycle and transmission dynamics of Pneumocystis jirovecii.

METHODS

We performed a cross sectional study of the prevalence and clinical predictors of Pneumocystis colonisation in 172 HIV infected, PCP negative inpatients undergoing diagnostic evaluation of 183 episodes of pneumonia at either the Medical Center of Louisiana at New Orleans between 2003 and 2005 or San Francisco General Hospital between 2000 and 2005. DNA was extracted from sputum and bronchoalveolar lavage specimens and amplified using a nested PCR assay at the mitochondrial large subunit (18S) ribosomal RNA locus. Colonisation was deemed present if Pneumocystis DNA was identified by both gel electrophoresis and direct DNA sequencing.

RESULTS

68% (117/172) of all patients were colonised with Pneumocystis. No strong associations with colonisation were identified for any demographic factors. Among clinical factors, having a CD4+ T cell count </=50 cells/mul (unadjusted OR 2.4, 95% CI 1.09 to 5.48; p = 0.031) and using PCP prophylaxis (unadjusted OR 0.55, 95% CI 0.29 to 1.07; p = 0.077) were associated with Pneumocystis colonisation, although the latter association may have been due to chance. After adjustment for CD4+ T cell count, use of PCP prophylaxis was associated with a decreased odds of colonisation (adjusted OR 0.45, 95% CI 0.21 to 0.98; p = 0.045). 11 patients who were colonised were subsequently readmitted for evaluation of a second episode of pneumonia; three were found to be colonised again, but none had PCP.

CONCLUSIONS

The majority of hospitalised HIV infected patients with non-PCP pneumonia are colonised with Pneumocystis. Failure to use co-trimoxazole prophylaxis and severe immunosuppression are associated with an increase in the odds of colonisation. Pneumocystis colonisation among hospitalised patients does not commonly lead to PCP.

Kinetics of Borrelia burgdorferi infection in larvae of refractory and competent tick vectors

The acquisition of Borrelia burgdorferi by the larvae of competent and refractory ixodid ticks was assessed by quantitative polymerase chain reaction (PCR). Larvae were fed on infected mice, and the spirochete loads were determined during feeding and up to 93 d postfeeding. Amblyomma americanum (L.) was refractory to B. burgdorferi infection, with almost no detection of spirochete DNA during or postfeeding. In contrast, Ixodes scapularis Say supported high loads of spirochetes (10(3)-10(4) per larva). In Dermacentor variabilis (Say), B. burgdorferi uptake was reduced, with an average of 16 spirochetes per larvae acquired after 4 d of feeding, representing 1/195 of the counts in I. scapularis. However, during the first day postfeeding, the spirochete growth rate in D. variabilis reached 0.076 generations per hour, 7.7 times greater than the highest growth rate detected in I. scapularis. D. variabilis supported intense spirochete growth up to the fourth day postinfection, when the counts increased to an average of 282 spirochetes per larvae or 1/8.5 of the I. scapularis counts 4 d postfeeding. The kinetics of spirochete growth was unstable in D. variabilis compared with I. scapularis, and transmission of B. burgdorferi by D. variabilis could not be demonstrated. A cofeeding experiment indicated that I. scapularis feeding increased A. americanum spirochete uptake. These collective results indicate suboptimal conditions for B. burgdorferi uptake and colonization within A. americanum or the presence of anti-Borrelia factor(s) in this nonpermissive tick species.

Capillary feeding of specific dsRNA induces silencing of the isac gene in nymphal Ixodes scapularis ticks

Ixodes scapularis transmits several pathogens including Borrelia burgdorferi. Bioactive compounds in tick saliva support tick feeding and influence pathogen transmission to the mammalian host. These studies utilized oral delivery of dsRNA to silence an anticomplement gene (isac) in I. scapularis nymphs. Silencing of isac significantly reduced fed-tick weight compared to delivery of control lacZ dsRNA, and immunoblots specific for FlaB protein indicated a reduction in spirochete load in isac-silenced infected nymphs. SDS-PAGE demonstrated that isac gene silencing affected expression of a number of salivary and non-salivary gland proteins in ticks. Finally, multiple isac cDNA homologues were cloned, and these may represent a new gene family coexpressed during tick feeding. This work presents a novel oral delivery approach for specific gene silencing in I. scapularis nymphs and characterizes the effect of isac on blood-feeding in an attempt to block transmission of B. burgdorferi.

Differential infectivity of the Lyme disease spirochete Borrelia burgdorferi derived from Ixodes scapularis salivary glands and midgut

Blood fed nymphal Ixodes scapularis Say infected with Borrelia burgdorferi were dissected to obtain salivary gland and midgut extracts. Extracts were inoculated into C3H/HeJ mice, and ear, heart, and bladder were cultured to determine comparative infectivity. Aliquots of extracts were then analyzed by quantitative polymerase chain reaction to determine the number of spirochetes inoculated into mice. A comparative median infectious dose (ID50) was determined for both salivary gland and midgut extract inoculations. Our data demonstrated a statistically significant difference (P < 0.002) in the ID50 derived from salivary gland (average = 18) versus midgut (average = 251) extracts needed to infect susceptible mice. A rationale for the differential infectivity of salivary and midgut derived spirochetes is discussed.

Effect of mutations in Pneumocystis carinii dihydropteroate synthase gene on outcome of P carinii pneumonia in patients with HIV-1: a prospective study

BACKGROUND

Investigators have reported that patients infected with Pneumocystis carinii containing mutations in the DHPS (dihydropteroate synthase) gene have a worse outcome than those infected with P carinii containing wild-type DHPS. We investigated patients with HIV-1 infection and P carinii pneumonia to determine if DHPS mutations were associated with poor outcomes in these patients.

METHODS

We compared presence of mutations at the DHPS locus with survival and response of patients to co-trimoxazole or other drugs.

FINDINGS

For patients initially given co-trimoxazole, nine (14%) of 66 with DHPS mutant died, compared with nine (25%) of 36 with wild type (risk ratio50.55 [95% CI=0.24-1.25]; p=0.15). Ten (15%) of 66 patients with a DHPS mutant did not respond to treatment, compared with 13 (36%) of 36 patients with the wild type (0.42 [0.20-0.86]; p=0.02). For patients aged 40 years or older, four (14%) of 29 with the mutant and nine (56%) of 16 with the wild type died (0.25 [0.09-0.67]; p=0.005).

INTERPRETATION

These results, by contrast with those of previous studies, suggest that patients with wild-type P carinii do not have a better outcome than patients with the mutant when given co-trimoxazole. Our results suggest that presence of a DHPS mutation should be only one of several criteria guiding the choice of initial drug treatment of P carinii pneumonia in patients with HIV-1 infection.

Genome size determination and coding capacity of Sodalis glossinidius, an enteric symbiont of tsetse flies, as revealed by hybridization to Escherichia coli gene arrays

Recent molecular characterization of various microbial genomes has revealed differences in genome size and coding capacity between obligate symbionts and intracellular pathogens versus free-living organisms. Multiple symbiotic microorganisms have evolved with tsetse fly, the vector of African trypanosomes, over long evolutionary times. Although these symbionts are indispensable for tsetse fecundity, the biochemical and molecular basis of their functional significance is unknown. Here, we report on the genomic aspects of the secondary symbiont Sodalis glossinidius. The genome size of Sodalis is approximately 2 Mb. Its DNA is subject to extensive methylation and based on some of its conserved gene sequences has an A+T content of only 45%, compared to the typically AT-rich genomes of endosymbionts. Sodalis also harbors an extrachromosomal plasmid about 134 kb in size. We used a novel approach to gain insight into Sodalis genomic contents, i.e., hybridizing its DNA to macroarrays developed for Escherichia coli, a closely related enteric bacterium. In this analysis we detected 1,800 orthologous genes, corresponding to about 85% of the Sodalis genome. The Sodalis genome has apparently retained its genes for DNA replication, transcription, translation, transport, and the biosynthesis of amino acids, nucleic acids, vitamins, and cofactors. However, many genes involved in energy metabolism and carbon compound assimilation are apparently missing, which may indicate an adaptation to the energy sources available in the only nutrient of the tsetse host, blood. We present gene arrays as a rapid tool for comparative genomics in the absence of whole genome sequence to advance our understanding of closely related bacteria.

Molecular tools and triatomine systematics: a public health perspective

Triatomines, or kissing bugs, are vectors of Chagas disease to humans. This disease is a substantial public health problem affecting up to 12 million people throughout the Americas, and its control relies mainly on the insecticide treatment of triatomine-infested houses within villages. In this article, Fernando Monteiro, Ananias Escalante and Ben Beard review how molecular markers have been used to clarify triatomine systematics, and give examples of how our understanding of triatomine population structure and accurate vector identification can be used to optimize vector control.

Sequence and organization of the mitochondrial genome of the Chagas disease vector, Triatoma dimidiata

The 17 019 bp mitochondrial genome of Triatoma dimidiata is composed of thirteen protein coding sequences, twenty-two tRNAs, small and large ribosomal units, and a control region. The gene order and orientation are identical to that of Drosophila yakuba. The nucleotide composition is biased toward adenine and thymine (69.5% A + T). The 2.1 kb putative control region, known as the A + T rich region in most insects, has an A + T bias of 66%, but contains a 400 bp sequence that is 77.5% A + T and two other distinct regions: (1) one with a lower A + T bias (60.1%) and (2) a region of eight tandem repeat units. The identified 1.4 kb nuclear copy of mitochondrial sequences encompasses the string of Gs and the beginning of the cytochrome c oxidase 1 gene but lacks the 1.8 kb region spanning the eight tandem repeats and the 5' end of the NADH dehydrogenase subunit II gene.

Bacterial symbiosis and paratransgenic control of vector-borne Chagas disease

The triatomine vectors of Chagas disease are obligate haematophagous insects, feeding on vertebrate blood throughout their entire developmental cycle. As a result of obtaining their nutrition from a single food source, their diet is devoid of certain vitamins and nutrients. Consequently, these insects harbour populations of bacterial symbionts within their intestinal tract, which provide the required nutrients that are lacking from their diet. We have isolated and characterised symbiont cultures from various triatomine species and developed a method for genetically transforming them. We can then reintroduce them into their original host species, thereby producing stable paratransgenic insects in which we are able to express heterologous gene products. Using this methodology, we have generated paratransgenic Rhodnius prolixus that are refractory for infection with Trypanosoma cruzi. Two examples of potentially refractory genes are currently being expressed in paratransgenic insects. These include the insect immune peptide cecropin A and active single chain antibody fragments. We have also developed an approach that would allow introduction of genetically modified bacterial symbionts into natural populations of Chagas disease vectors. This approach utilises the coprophagic behaviour of these insects, which is the way in which the symbionts are transmitted among bug populations in nature. The production and ultimate release of transgenic or paratransgenic insects for public health applications is potentially very promising but also worthy of much careful consideration with respect to environmental, political, and human safety concerns.

Pneumocystis carinii cytochrome b mutations are associated with atovaquone exposure in patients with AIDS

This retrospective cohort study was conducted to determine whether Pneumocystis carinii cytochrome b gene mutations in patients with AIDS and P. carinii pneumonia (PCP) are associated with atovaquone exposure. Portions of the P. carinii cytochrome b genes that were obtained from 60 patients with AIDS and PCP from 6 medical centers between 1995 and 1999 were amplified and sequenced by using polymerase chain reaction. Fifteen patients with previous atovaquone prophylaxis or treatment exposure were matched with 45 patients with no atovaquone exposure. Cytochrome b coenzyme Q binding site mutations were observed in 33% of isolates from patients exposed to atovaquone, compared with 6% from those who were not (P=.018). There was no difference in survival 1 month after treatment between patients with or without cytochrome b mutations (P=.14). Thus, cytochrome b mutations are significantly more common in patients with AIDS and PCP with atovaquone exposure, but the clinical significance of these mutations remains unknown.

Sulfa or sulfone prophylaxis and geographic region predict mutations in the Pneumocystis carinii dihydropteroate synthase gene

To determine factors associated with mutations in the Pneumocystis carinii dihydropteroate synthase (DHPS) gene, a prospective study of human immunodeficiency virus (HIV)-infected patients with confirmed P. carinii pneumonia was conducted in Atlanta, Seattle, and San Francisco. Clinical information was obtained from patient interview and chart abstraction. DHPS genotype was determined from DNA sequencing. Overall, 76 (68.5%) of 111 patients had a mutant DHPS genotype, including 22 (81.5%) of 27 patients from San Francisco. In multivariate analysis, sulfa or sulfone prophylaxis and study site were independent predictors of a mutant genotype. Fourteen (53.8%) of 26 patients who were newly diagnosed with HIV infection and had never taken prophylaxis had a mutant genotype. The significance of geographic location as a risk factor for mutant genotype and the high proportion of mutant genotypes among persons never prescribed prophylaxis, including those newly diagnosed with HIV infection, provide indirect evidence that these mutations are transmitted from person to person either directly or through a common environmental source.

Mitochondrial DNA variation of Triatoma infestans populations and its implication on the specific status of T. melanosoma

DNA sequence comparison of 412 base-pairs fragments of the mitochondrial cytochrome B gene was used to infer the genetic structure of nine geographical Triatoma infestans populations and their phylogenetic relationship with T. melanosoma and T. brasiliensis. T. infestans and T. melanosoma were compared by morphometry, allozyme and cytogenetic analyses, as well as subjected to reciprocal crosses, in order to clarify the taxonomic status of the latter. No differences were found to distinguish the two species and the crosses between them yielded progeny. T. infestans populations presented four haplotypes that could be separated in two clusters: one formed by the samples from Bolivia (Andes and Chaco) and the other formed by samples from Argentina and Brazil. Silvatic and domestic T. infestans populations from Bolivia (Andes) were genetically identical.

Molecular epidemiology of metronidazole resistance in a population of Trichomonas vaginalis clinical isolates

Trichomonas vaginalis, the causative agent for human trichomoniasis, is a problematic sexually transmitted disease mainly in women, where it may be asymptomatic or cause severe vaginitis and cervicitis. Despite its high prevalence, the genetic variability and drug resistance characteristics of this organism are poorly understood. To address these issues, genetic analyses were performed on 109 clinical isolates using three approaches. First, two internal transcribed spacer (ITS) regions flanking the 5.8S subunit of the ribosomal DNA gene were sequenced. The only variation was a point mutation at nucleotide position 66 of the ITS1 region found in 16 isolates (14.7%). Second, the presence of a 5.5-kb double-stranded RNA T. vaginalis virus (TVV) was assessed. TVV was detected in 55 isolates (50%). Finally, a phylogenetic analysis was performed based on random amplified polymorphic DNA data. The resulting phylogeny indicated at least two distinct lineages that correlate with the presence of TVV. A band-sharing index indicating relatedness was created for different groups of isolates. It demonstrated that isolates harboring the virus are significantly more closely related to each other than to the rest of the population, and it indicated a high level of relatedness among isolates with in vitro metronidazole resistance. This finding is consistent with the hypothesis that drug resistance to T. vaginalis resulted from a single or very few mutational events. Permutation tests and nonparametric analyses showed associations between metronidazole resistance and phylogeny, the ITS mutation, and TVV presence. These results suggest the existence of genetic markers with clinical implications for T. vaginalis infections.

Research ethics. Managing risks of arthropod vector research

Field research with vectors is an essential aspect of vector biology research and vector-borne disease prevention and control. This type of research, which brings experimental vector manipulations into endemic areas, can present risks to human populations. This paper seeks to stimulate a full discussion within the medical entomology community of the risks associated with vector field research. Such discussions will promote development of a consensus, among investigators, sponsoring agencies and the communities within which the work is done, so that appropriate steps can be taken to minimize and manage the risks, and adequate oversight can be maintained.

Phylogeny and molecular taxonomy of the Rhodniini derived from mitochondrial and nuclear DNA sequences

Eleven species of Rhodnius and one of Psammolestes were compared by DNA sequence analysis of fragments of the mitochondrial large subunit ribosomal RNA (mtlsurRNA), the mitochondrial cytochrome b (mtCytb), and the D2 variable region of the 28S nuclear RNA (D2), totaling 1,429 base pairs. The inferred phylogeny, using Triatoma infestans as an outgroup, revealed two main clades within the Rhodniini--one, including the prolixus group of species (Rhodnius prolixus, Rhodnius robustus, Rhodnius neglectus, and Rhodnius nasutus) together with Rhodnius domesticus and Rhodnius neivai, and the other comprising two groups formed by Rhodnius pictipes plus Rhodnius brethesi, and Rhodnius ecuadoriensis plus Rhodnius pallescens. Psammolestes tertius appeared most closely related to the prolixus group. The analysis strongly supports the validity of R. robustus as a species distinct from others of the prolixus group, but suggests higher genetic structuring of R. robustus populations compared to the other species. Although R. robustus has been found naturally infected by Trypanosoma cruzi, the fact that it is apparently entirely sylvatic and unable to establish in homes suggests that it is of no great importance as a Chagas disease vector in humans.

Genetic variation in Pneumocystis carinii isolates from different geographic regions: implications for transmission

To study transmission patterns of Pneumocystis carinii pneumonia (PCP) in persons with AIDS, we evaluated P. carinii isolates from patients in five U.S. cities for variation at two independent genetic loci, the mitochondrial large subunit rRNA and dihydropteroate synthase. Fourteen unique multilocus genotypes were observed in 191 isolates that were examined at both loci. Mixed infections, accounting for 17.8% of cases, were associated with primary PCP. Genotype frequency distribution patterns varied by patients' place of diagnosis but not by place of birth. Genetic variation at the two loci suggests three probable characteristics of transmission: that most cases of PCP do not result from infections acquired early in life, that infections are actively acquired from a relatively common source (humans or the environment), and that humans, while not necessarily involved in direct infection of other humans, are nevertheless important in the transmission cycle of P. carinii f. sp. hominis.

Molecular epidemiology of cryptosporidiosis outbreaks and transmission in British Columbia, Canada

Isolates from 25 (13 sporadic and 12 outbreak) cryptosporidiosis cases, 24 of which were from British Columbia, Canada, were characterized using nested polymerase chain reaction amplification of the polymorphic internal transcribed spacer 1 locus. Two predominant Cryptosporidium parvum genotypes were found. Twelve (8 sporadic and 4 outbreak) isolates amplified with the cry7/cry21 primer pair and 12 (5 sporadic and 7 outbreak) isolates amplified with the cry7/cryITS1 primer pair. Multi-locus gene analysis using sequence polymorphisms on 3 other loci, i.e., the thrombospondin-related adhesion protein gene, the dihydrofolate reductase gene, and the 18S rRNA gene on 8 (4 outbreak and 4 sporadic) isolates showed non-random association among the human and animal alleles of the 4 different C. parvum gene loci. Associations between these 2 parasite genotypes and different routes of cryptosporidiosis transmission such as zoonotic, anthroponotic, and waterborne transmission were studied using municipal population and agricultural information, as well as detection of C. parvum oocysts in municipal drinking water specimens of the residential communities of sporadic and outbreak cases.

Expression of a functional antibody fragment in the gut of Rhodnius prolixus via transgenic bacterial symbiont Rhodococcus rhodnii

Expression within insects of foreign antiparasitic gene products via microbial symbionts could be used to prevent transmission of vector-borne pathogens to vertebrate hosts. Genetically transformed symbiotic bacteria Rhodococcus rhodnii expressed functional antibody fragments (rDB3 encoding murine V(H)/K which binds progesterone) that were exported into the gut lumen of the triatomine bug Rhodnius prolixus (Hemiptera: Reduviidae), a vector of Chagas disease. Transgenic symbionts were maintained in successive nymphal instars and adults of Rhodnius prolixus despite competition with native untransformed Rhodococcus rhodnii. This is the first description of a functional mammalian antibody fragment expressed in an insect. Our system is a model for constructing paratransgenic insects (insects carrying transformed symbionts) with compromised ability to transmit pathogens.

Mitochondrial DNA sequence variation among triatomine vectors of Chagas' disease

Kissing bugs or triatomines (Reduviidae: Triatominae) are vectors of the Chagas' disease agent Trypanosoma cruzi. There is a current need for more sensitive tools for use in discrimination of different bug populations and species, thus allowing a better understanding of these insects as it relates to disease transmission and control. In a preliminary analysis of the mitochondrial large subunit ribosomal RNA (mtlsurRNA) and cytochrome B (mtCytB) genes, we used DNA sequencing to study species identification and phylogeny. In both examined gene regions, about 46% of nucleotide positions exhibited polymorphism. The examined region of mtCytB appears to have evolved more rapidly than the examined region of mtlsurRNA. Phylogenetic analysis of both gene fragments in the examined species produced similar results that were generally consistent with the accepted taxonomy of the subfamily. The two major tribes, Rhodniini and Triatomini, were supported, along with additional clades that corresponded to accepted species complexes within the Rhodnius and Triatoma genera. The one chief exception was that Psammolestes coreodes sorted into the Rhodnius prolixus-robustus-neglectus clade, with bootsrap values of 99% and 81%, respectively, for the mtlsurRNA and mtCytB fragments. All of the individual species examined could be distinguished at both genetic loci.

Tsetse thrombin inhibitor: bloodmeal-induced expression of an anticoagulant in salivary glands and gut tissue of Glossina morsitans morsitans

The tsetse thrombin inhibitor, a potent and specific low molecular mass (3,530 Da) anticoagulant peptide, was purified previously from salivary gland extracts of Glossina morsitans morsitans (Diptera: Glossinidae). A 303-bp coding sequence corresponding to the inhibitor has now been isolated from a tsetse salivary gland cDNA library by using degenerate oligonucleotide probes. The full-length cDNA contains a 26-bp untranslated segment at its 5' end, followed by a 63-bp sequence corresponding to a putative secretory signal peptide. A 96-bp segment codes for the mature tsetse thrombin inhibitor, whose predicted molecular weight matches that of the purified native protein. Based on its lack of homology to any previously described family of molecules, the tsetse thrombin inhibitor appears to represent a unique class of naturally occurring protease inhibitors. Recombinant tsetse thrombin inhibitor expressed in Escherichia coli and the chemically synthesized peptide are both substantially less active than the purified native protein, suggesting that posttranslational modification(s) may be necessary for optimal inhibitory activity. The tsetse thrombin inhibitor gene, which is present as a single copy in the tsetse genome, is expressed at high levels in salivary glands and midguts of adult tsetse flies, suggesting a possible role for the anticoagulant in both feeding and processing of the bloodmeal.

Rickettsial pathogens and their arthropod vectors

Rickettsial diseases, important causes of illness and death worldwide, exist primarily in endemic and enzootic foci that occasionally give rise to sporadic or seasonal outbreaks. Rickettsial pathogens are highly specialized for obligate intracellular survival in both the vertebrate host and the invertebrate vector. While studies often focus primarily on the vertebrate host, the arthropod vector is often more important in the natural maintenance of the pathogen. Consequently, coevolution of rickettsiae with arthropods is responsible for many features of the host-pathogen relationship that are unique among arthropod-borne diseases, including efficient pathogen replication, long-term maintenance of infection, and transstadial and transovarial transmission. This article examines the common features of the host-pathogen relationship and of the arthropod vectors of the typhus and spotted fever group rickettsiae.

Differentiating human from animal isolates of Cryptosporidium parvum

We analyzed 92 Cryptosporidium parvum isolates from humans and animals by a polymerase chain reaction/restriction fragment length polymorphism method based on the thrombospondin-related anonymous protein 2 gene sequence. Used as a molecular marker, this method can differentiate between the two genotypes of C. parvum and elucidate the transmission of infection to humans.

Bacterial symbiosis in arthropods and the control of disease transmission

Bacterial symbionts may be used as vehicles for expressing foreign genes in arthropods. Expression of selected genes can render an arthropod incapable of transmitting a second microorganism that is pathogenic for humans and is an alternative approach to the control of arthropod-borne diseases. We discuss the rationale for this alternative approach, its potential applications and limitations, and the regulatory concerns that may arise from its use in interrupting disease transmission in humans and animals.

Prevention of insect-borne disease: an approach using transgenic symbiotic bacteria

Expression of molecules with antiparasitic activity by genetically transformed symbiotic bacteria of disease-transmitting insects may serve as a powerful approach to control certain arthropod-borne diseases. The endosymbiont of the Chagas disease vector, Rhodnius prolixus, has been transformed to express cecropin A, a peptide lethal to the parasite, Trypanosoma cruzi. In insects carrying the transformed bacteria, cecropin A expression results in elimination or reduction in number of T. cruzi. A method has been devised to spread the transgenic bacteria to populations of R. prolixus, in a manner that mimics their natural coprophagous route of symbiont acquisition.

Genetic polymorphism among Cryptosporidium parvum isolates: evidence of two distinct human transmission cycles

We report the results of molecular analysis of 39 isolates of Cryptosporidium parvum from human and bovine sources in nine human outbreaks and from bovine sources from a wide geographic distribution. All 39 isolates could be divided into either of two genotypes, on the basis of genetic polymorphism observed at the thrombospondin-related adhesion protein (TRAP-C2) locus. Genotype 1 was observed only in isolates from humans. Genotype 2, however, was seen in calf isolates and in isolates from a subset of human patients who reported direct exposure to infected cattle or consumed items thought to be contaminated with cattle faces. Furthermore, experimental infection studies showed that genotype 2 isolates were infective to mice or calves under routine laboratory conditions, whereas genotype 1 isolates were not. These results support the occurrence of two distinct transmission cycles of C. parvum in humans.

The polyubiquitin gene of the mosquito Anopheles gambiae: structure and expression

The polyubiquitin gene from the mosquito Anopheles gambiae has been cloned and sequenced, and its structure is reported along with sequence analysis results. The gene consists of approximately seven tandem head-to-tail repeat units of the seventy-six amino acid-coding ubiquitin monomer. It is expressed constitutively in larvae, pupae and adults of An. gambiae, as well as in a cell line derived from this mosquito species. A probe made from a DNA fragment containing the coding region of the gene recognizes transcripts of approximately 3.6 kb and 4.4 kb in RNA isolated from all mosquito developmental stages and a unique transcript of approximately 3.0 kb in RNA from the cell line. Single monomeric units of the An. gambiae polyubiquitin gene shared from 75.9% to 85.5% identity at the DNA level with homologous sequences from other organisms ranging from yeast to man. A comparison of individual repeat units of the An. gambiae gene revealed that, in general, the 5' ends of the individual monomers are more highly conserved than the 3' ends. The gene mapped by in situ hybridization on ovarian nurse cell polytene chromosomes to a primary site at division 12C on chromosome 2R and to a secondary site at division 9C on the same chromosome.

Eye pigments in wild-type and eye-color mutant strains of the African malaria vector Anopheles gambiae

Chromatographic analysis of pigments extracted from wild-type eyes of the mosquito Anopheles gambiae reveals the presence of the ommatin precursor 3-hydroxykynurenine, its transamination derivative xanthurenic acid, and a dark, red-brown pigment spot that probably is composed of two or more low mobility xanthommatins. No colored or fluorescent pteridines are evident. Mosquitoes homozygous for an autosomal recessive mutation at the red-eye (r) locus have a brick-red eye color in larvae, pupae, and young adults, in contrast to the almost black color of the wild eye. Mosquitoes homozygous for this mutant allele have levels of ommochrome precursors that are indistinguishable from the wild-type, but the low-mobility xanthommatin spot is ochre-brown in color rather than red-brown as in the wild-type. Mosquitoes with two different mutant alleles at the X-linked pink-eye locus (p, which confers a pink eye color, and pw, which confers a white eye phenotype in homozygotes or hemizygous males) have normal levels of ommochrome precursors but no detectable xanthommatins. Mosquitoes homozygous for both the r and p mutant alleles have apricot-colored eyes and show no detectable xanthommatins. Both the pink-eye and red-eye mutations appear to involve defects in the transport into or assembly of pigments in the membrane-bound pigment granules rather then defects in ommochrome synthesis.

A cytoskeletal actin gene in the mosquito Anopheles gambiae

Five actin genes have been identified in the mosquito Anopheles gambiae, and a constitutively expressed actin gene has been chosen for detailed analysis. We have physically mapped and sequenced this gene and six associated cDNAs, including translated coding regions, as well as the 5' and 3' flanking sequences. Analysis of stage-specific RNA shows this gene to be present in all stages of mosquito development and in an established A. gambiae cell line, thus indicating a cytoskeletal actin. In the sequence of the translated coding region and in pattern of expression, this gene is very similar to the cytoskeletal actin genes of Drosophila melanogaster, and in sequence, equally similar to the Artemia cytoskeletal actin gene 403 (99.2% identity among the three amino acid sequences). Sequencing of this A. gambiae actin gene (designated act1D for its location in chromosome division 1D) and selected cDNAs shows that it possesses three alternative leader sequences; thus the gene appears to have three alternative promoters. These promoters should ultimately prove useful in the production of transgenic constructs for constitutive expression.

A cDNA encoding an ADP/ATP carrier from the mosquito Anopheles gambiae

Two cDNAs are described from Anopheles gambiae that correspond to the ADP/ATP carrier or translocase. The clones are identical except for minor differences in the 5' non-coding region and in the lengths of the poly-A tails. They code for mRNAs of 1261 and 1263 bp and contain one open reading frame of 906 bp. A probe made from the 1263 bp cDNA hybridized to bands of approximately 1260 and 1700 bp on developmental Northern blots. The putative 300 amino acid peptide sequence shows from 53.4-78.5% identity to AAC peptide sequences from a range of organisms from Zea mays to human. Both clones mapped to region 26a on the left arm of chromosome 2 in An. gambiae.

Modification of arthropod vector competence via symbiotic bacteria

Some of the world's most devastating diseases are transmitted by arthropod vectors. Attempts to control these arthropods are currently being challenged by the widespread appearance of insecticide resistance. It is therefore desirable to develop alternative strategies to complement existing methods of vector control. In this review, Charles Beard, Scott O'Neill, Robert Tesh, Frank Richards and Serap Aksoy present an approach for introducing foreign genes into insects in order to confer refractoriness to vector populations, ie. the inability to transmit disease-causing agents. This approach aims to express foreign anti-parasitic or anti-viral gene products in symbiotic bacteria harbored by insects. The potential use of naturally occurring symbiont-based mechanisms in the spread of such refractory phenotypes is also discussed.

Genetic transformation and phylogeny of bacterial symbionts from tsetse

Two isolates of bacterial endosymbionts, GP01 and GM02, were established in cell free medium from haemolymph of the tsetse, Glossina pallidipes and G. morsitans. These microorganisms appear similar to rickettsia-like organisms reported previously from various tsetse species. The 16S rRNA sequence analysis, however, placed them within the gamma subdivision of the Proteobacteria, phylogenetically distinct from most members of the Rickettsiaceae which align with the alpha subdivision. Distinct multiple endogenous plasmids are harboured by GP01 and GM02, suggesting that the two isolates are different. Restriction mapping analysis showed that one of the conserved plasmids is present in high copy number and is at least 80 kb in size. A heterologous plasmid pSUP204, which contains the broad host range oriV replication origin, was used to transfect bacterial cultures. The symbiont GM02 was transformed, and it expressed plasmid encoded resistance to the antibiotics ampicillin, tetracycline and chloramphenicol. Transformation of these symbionts may provide a novel means for expressing anti-parasitic genes within tsetse populations.

The mitochondrial genome of the mosquito Anopheles gambiae: DNA sequence, genome organization, and comparisons with mitochondrial sequences of other insects

The entire 15,363 bp mitochondrial genome was cloned and sequenced from the mosquito Anopheles gambiae. With respect to the protein-coding genes, rRNA genes and the control region, the gene order was identical to that reported for other insects. There were significant differences, however, in the position and orientation of specific tRNA loci. The overall nucleotide composition was heavily biased towards adenine and thymine, which accounted for 77.6% of all nucleotides. Comparisons were made with the mitochondrial genomes of other insects on the basis genome size and organization, DNA and putative amino acid sequence data, nucleotide substitutions, codon usage and bias, and patterns of AT enrichment.

Spliced leader RNA sequences of Trypanosoma rangeli are organized within the 5S rRNA-encoding genes

The spliced leader RNA(SL RNA)-encoding genes of the salivarian New World trypanosome, Trypanosoma rangeli, are organized within the 5S rRNA tandem repeats. Each repeat contains genes encoding an SL RNA and a 5S rRNA in the same orientation of transcription. This SL-5S organization is also present in the African trypanosome, Trypanosoma vivax. A similar association of SL and 5S genes has been observed in some nematodes, but has not been described previously in trypanosomatids.

Transformation of an insect symbiont and expression of a foreign gene in the Chagas' disease vector Rhodnius prolixus

A shuttle plasmid was developed that is capable of replicating both in Escherichia coli and in Rhodococcus rhodnii, a bacterial symbiont of the Chagas' disease vector Rhodnius prolixus. We have been able to transform R. rhodnii with this plasmid, infect aposymbiotic R. prolixus with the transformed symbionts, select with the antibiotic thiostrepton, and re-isolate genetically altered symbionts from the insects following successive molts. Symbiotic bacteria are potentially valuable as vehicles for the stable introduction of foreign genes into insects with the goal of eventually altering the ability of the insect to transmit a pathogenic agent.

A new member of a family of site-specific retrotransposons is present in the spliced leader RNA genes of Trypanosoma cruzi

A new member of a family of site-specific retrotransposons is described in the New World trypanosome Trypanosoma cruzi. This element, CZAR (cruzi-associated retrotransposon), resembles two previously described retrotransposons found in the African trypanosome T. brucei gambiense and the mosquito trypanosomatid Crithidia fasciculata in specifically inserting between nucleotides 11 and 12 of the highly conserved 39-mer of the spliced leader RNA (SL-RNA) gene. CZAR is similar in overall organization to the other two SL-RNA-associated elements. It possesses two potential long open reading frames which resemble the gag and pol genes of retroviruses. In the pol open reading frame, all three elements contain similarly arranged endonuclease domains and share extensive amino acid homology in the reverse transcriptase region. All are associated with the SL-RNA gene locus and are present in low copy numbers. They do not appear to have 5' truncated versions. All three retrotransposons are otherwise quite distinct from one another, with no significant overall amino acid homology. The presence of such retroelements inserted into the identical site within SL-RNA gene sequences in at least three evolutionarily distant trypanosomatid species argues for a functional role. Because these elements appear to have a precise target site requirement for integration, we refer to them as SL siteposons.

Nolleria pulicis n. gen., n. sp. (Microsporida: Chytridiopsidae), a microsporidian parasite of the cat flea, Ctenocephalides felis (Siphonaptera: Pulicidae)

A new species of microsporidium, Nolleria pulicis, is described and named here from the cat flea, Ctenocephalides felis. The genus Nolleria is created and placed within the family Chytridiopsidae. The family is slightly modified to accommodate certain features of intracellular development seen in N. pulicis, which is otherwise very similar to other species in the family Chytridiopsidae. Sporulation is described from ultrastructural analysis of infected midgut epithelial cells of adult C. felis. The term "multiple division by vacuolation" is proposed for describing sporogony as it occurs in this species and certain related species of microsporidia. The probable mode of transmission and apparent absence of merogony are discussed.

First isolation of Trypanosoma cruzi from a wild-caught Triatoma sanguisuga (LeConte) (Hemiptera: Triatominae) in Florida, U.S.A

An adult female Triatoma sanguisuga was found to be naturally infected with Trypanosoma cruzi. This is the first report of a T. cruzi infection in this bug in Florida and suggests that a study of trypanosomiasis in reservoirs and vectors in north Florida is warranted.

HNMR of succinate binding to aspartate transcarbamylase. A comparison of results in D2O and H2O

The interaction of succinate with asparatete transcarbamylase from Escherichia coli has been studied by magnetic resonance relaxation measurements of the dicarboxylic acid methylene protons in H2O solutions. The pH and temperature dependence of the relaxation in the presence of either native asparte transcarbamylase or its catalytic subunit in H2O solutions is qualitatively very similar to the corresponding situation utilizing D2O as the solvent. From previous result of measurements in D2O[C.B. Beard and P.G. Schmidt, Biochemistry 12(1973)2255] a mechanism was proposed involving 2 protonated groups affecting succinate binding and titratable over the pH range 7-10. Quantitatively, fitting the data from H2O solutions to the mechanism yeilds values of the fitting parameters generally in good agreement with the D2O experiments. The main exceptions are the pKa values calculated for the two titratable groups. For these species the values obtained in the presence of the catalytic subunit are 6.7 and 7.8 in H2O solutions versus 7.3 and 8.6 in D2O solutions. In the presence of native enzyme the corresponding values are 6.8 and 8.3 in H2O versus 7.6 and 9.2 in D2O. These observed differences are consistent with differences in ionization constants of weak acids in D2O relative to H2O. The results imply that succinate interaction with the enzyme active site is similar in the two solvents.