Biomedical subjects

R Karunakaran

Publications and source records attributed to R Karunakaran.

6 recordsLinked to original sources

PubMed2006

Thiamine is synthesized by a salvage pathway in Rhizobium leguminosarum bv. viciae strain 3841.

In the absence of added thiamine, Rhizobium leguminosarum bv. viciae strain 3841 does not grow in liquid medium and forms only "pin" colonies on agar plates, which contrasts with the good growth of Sinorhizobium meliloti 1021, Mesorhizobium loti 303099, and Rhizobium etli CFN42. These last three organisms have thiCOGE genes, which are essential for de novo thiamine synthesis. While R. leguminosarum bv. viciae 3841 lacks thiCOGE, it does have thiMED. Mutation of thiM prevented formation of pin colonies on agar plates lacking added thiamine, suggesting thiamine intermediates are normally present. The putative functions of ThiM, ThiE, and ThiD are 4-methyl-5-(beta-hydroxyethyl) thiazole (THZ) kinase, thiamine phosphate pyrophosphorylase, and 4-amino-5-hydroxymethyl-2-methyl pyrimidine (HMP) kinase, respectively. This suggests that a salvage pathway operates in R. leguminosarum, and addition of HMP and THZ enabled growth at the same rate as that enabled by thiamine in strain 3841 but elicited no growth in the thiM mutant (RU2459). There is a putative thi box sequence immediately upstream of the thiM, and a gfp-mut3.1 fusion to it revealed the presence of a promoter that is strongly repressed by thiamine. Using fluorescent microscopy and quantitative reverse transcription-PCR, it was shown that thiM is expressed in the rhizosphere of vetch and pea plants, indicating limitation for thiamine. Pea plants infected by RU2459 were not impaired in nodulation or nitrogen fixation. However, colonization of the pea rhizosphere by the thiM mutant was impaired relative to that of the wild type. Overall, the results show that a thiamine salvage pathway operates to enable growth of Rhizobium leguminosarum in the rhizosphere, allowing its survival when thiamine is limiting.

Alkyl and Aryl Transferases↗

PubMed2006

Rhodococcus equi--an emerging human pathogen in immunocompromized hosts: a report of four cases from Malaysia.

Rhodococcus equi, a recognized pathogen in horses, is emerging as a human opportunistic pathogen, especially in immunocompromized hosts. We describe four immunocompromized patients who had serious R. equi infections with an overall mortality of 75%. The natural habitat of R. equi is soil, particularly soil contaminated with animal manure. Necrotizing pneumonia is the commonest form of infection but extrapulmonary infections, such as wound infections and subcutaneous abscess, have also been described in humans. R. equi is cultured easily in ordinary non-selective media. Large, smooth, irregular colonies appear within 48 hours. It is a facultative, intracellular, nonmotile, non-spore forming, gram-positive coccobacillus, which is weakly acid-fast staining and bears a similarity to diphtheroids. It forms a salmon-colored pigment usually after 48 hours incubation. A particular characteristic of this organism is that it undergoes synergistic hemolysis with some bacteria on sheep blood agar. R. equi may be misidentified as diphtheroids, Mycobacterium species, or Nocardia. In vitro R. equi is usually susceptible to erythromycin, ciprofloxacin, vancomycin, aminoglycosides, rifampin, imipenem and meropenem. The organism can be difficult to eradicate, making treatment challenging. Increased awareness of the infection may help with early diagnosis and timely treatment.

Actinomycetales Infections↗

PubMed2005

Community-acquired vancomycin-resistant Enterococcus faecium: a case report from Malaysia.

Vancomycin-resistant enterococci (VRE) are formidable organisms renowned for their ability to cause infections with limited treatment options and their potential for transferring resistance genes to other Gram-positive bacteria. Usually associated with nosocomial infections, VRE are rarely reported as a cause of community-acquired infection. Presented here is a case of community-acquired infection due to vancomycin-resistant Enterococcus faecium. The patient had been applying herbal leaves topically to his cheek to treat a buccal space abscess, resulting in a burn of the overlying skin. From pus aspirated via the skin a pure culture of E. faecium was grown that was resistant to vancomycin with a MIC of >256 microg ml-1 by the E test and resistant to teicoplanin by disc diffusion, consistent with the VanA phenotype. The organism was suspected of contaminating the leaf and infecting the patient via the burnt skin. This case highlights the need for further studies on the community prevalence of VRE among humans and animals to define unrecognized silent reservoirs for VRE, which may pose a threat to public health.

Abscess↗

PubMed2005

A family of promoter probe vectors incorporating autofluorescent and chromogenic reporter proteins for studying gene expression in Gram-negative bacteria.

A series of promoter probe vectors for use in Gram-negative bacteria has been made in two broad-host-range vectors, pOT (pBBR replicon) and pJP2 (incP replicon). Reporter fusions can be made to gfpUV, gfpmut3.1, unstable gfpmut3.1 variants (LAA, LVA, AAV and ASV), gfp+, dsRed2, dsRedT.3, dsRedT.4, mRFP1, gusA or lacZ. The two vector families, pOT and pJP2, are compatible with one another and share the same polylinker for facile interchange of promoter regions. Vectors based on pJP2 have the advantage of being ultra-stable in the environment due to the presence of the parABCDE genes. As a confirmation of their usefulness, the dicarboxylic acid transport system promoter (dctA(p)) was cloned into a pOT (pRU1097)- and a pJP2 (pRU1156)-based vector and shown to be expressed by Rhizobium leguminosarum in infection threads of vetch. This indicates the presence of dicarboxylates at the earliest stages of nodule formation.

Bacterial Proteins↗

PubMed2003

Amino-acid cycling drives nitrogen fixation in the legume-Rhizobium symbiosis.

The biological reduction of atmospheric N2 to ammonium (nitrogen fixation) provides about 65% of the biosphere's available nitrogen. Most of this ammonium is contributed by legume-rhizobia symbioses, which are initiated by the infection of legume hosts by bacteria (rhizobia), resulting in formation of root nodules. Within the nodules, rhizobia are found as bacteroids, which perform the nitrogen fixation: to do this, they obtain sources of carbon and energy from the plant, in the form of dicarboxylic acids. It has been thought that, in return, bacteroids simply provide the plant with ammonium. But here we show that a more complex amino-acid cycle is essential for symbiotic nitrogen fixation by Rhizobium in pea nodules. The plant provides amino acids to the bacteroids, enabling them to shut down their ammonium assimilation. In return, bacteroids act like plant organelles to cycle amino acids back to the plant for asparagine synthesis. The mutual dependence of this exchange prevents the symbiosis being dominated by the plant, and provides a selective pressure for the evolution of mutualism.

Amino Acids↗

PubMed2002

A retrospective review of mucocutaneous infections by human herpesvirus 1 and 2 in an urban population in Malaysia.

This is a 10-year retrospective review of mucocutaneous infection by human herpesvirus 1 (HHV1) and human herpesvirus 2 (HHV2) carried out by the virus diagnostic unit of University Malaya Medical Centre (UMMC). A total of 504 specimens from UMMC and a private clinic in the same city (KLSC) were tested; 198 samples from patients with oral lesions and 306 from patients with genital lesions. HHV1 was found to be responsible for 98.4% of oral lesions whereas HHV2 was the cause of 83.6% of all genital lesions. Detailed analysis showed no statistical difference by age group, race or gender among the patients with oral and genital lesions. Two laboratory methods were used in this study. Of the total 504 specimens tested, 18.0% specimens were positive by direct immunofluorescence (IF), 55.0% by virus isolation and 56.5% when both methods were used in combination. Although IF can provide a more rapid diagnosis, it is, however, less sensitive and can be attributed partly to inadequate collection of specimens.

Adolescent↗