Detection of acid-fast bacilli in postlysis debris of clinical specimens improves the reliability of PCR

Summer mortality syndrome bacterial pathogens in farmed Nile tilapia (Oreochromis niloticus)

Background

The high summer mortality in many fish farms, which had detrimental economic and social implications, was a serious challenge that the fish industry had to deal with.

Aim

With an examination of the most effective antibiotic, the ongoing research was intended to shed light on the identification of the main bacterial pathogens associated with the summer mortality syndrome in the diseased farmed Nile tilapia.

Methods

Six hundred dead Nile tilapia samples that had suffered from summer mortality were collected from several fish farms between May and October of 2022. The gathered fish displayed hemorrhagic areas on the skin, scale detachment, fin degeneration, erosions, skin ulcers, and corneal opacity with unilateral and/or bilateral exophthalmia. The most prominent internal appearance was swelling of the internal organs with sanguineous ascetic fluid.

Results

There were 225 bacterial isolates found. Six species were identified through phenotypic and biochemical analysis; they were Aeromonas, Vibrio, Streptococcus, Pseudomonas, Enterococcus, and Edwardsiella spp., in descending percentage, respectively. Aeromonas spp., Vibrio spp., and Streptococcus spp. were the three most frequent isolated bacterial pathogens. The identification of Aeromonas hydrophila, Vibrio spp., and Streptococcus iniae, the three most common bacterial isolates, was confirmed by molecular analysis by polymerase chain reaction. Most of the tested strains were found to be responsive to Ciprofloxacin (CIP), Gentamicin (CN), and Chloramphenicol (C) but resistant to Amoxicillin (AMX), according to an antibiotic sensitivity test.

Conclusion

The three most dangerous common bacterial infections discovered during mass-farmed tilapia summer mortality are A. hydrophil a, Vibrio sp., and S. iniae. This makes it clear that high water temperatures may raise the possibility of bacterial infections, which could cause widespread tilapia mortality and substantial financial losses. Therefore, it is crucial to maintain a beneficial fish culture, environment, and husbandry practices to enhance the tilapia-rearing environment and lessen the virulence of the disease. Isolated bacterial strains showed low levels of resistance to AMX but were vulnerable to CIP, CN, and C.

Detection of hemolytic strains of Aeromonas hydrophila and A . sobria along with other Aeromonas spp. from fish and fishery products by multiplex PCR

Hemolytic strains of Aeromonas spp. from fish and fishery products were detected by multiplex PCR. The selected primers for the amplification of segments of ahh1, asa1 and 16S rRNA gene yielded products with the size of 130 bp, 249 bp and 356 bp, respectively. This assay was found to be highly sensitive, as it could detect 7 and 9 cells of Aeromonas hydrophila and A. sobria with a detection limit of 1 pg of pure genomic DNA. The assay, when screened for 73 commercial fish and fishery product samples consisting of freshwater, marine fish and shellfish, showed 56 % positive for Aeromonas spp., 16 % for Aeromonas hydrophila and 13 % for A. sobria. This assay provides specific and reliable results and can be a powerful tool for the simultaneous detection of hemolytic strains of A. hydrophila A. sobria and other Aeromonas spp. from fish and fishery products.

Efficient diagnosis of tuberculous meningitis by detection of Mycobacterium tuberculosis DNA in cerebrospinal fluid filtrates using PCR

Tuberculous meningitis (TBM) is the most devastating form of meningitis and prompt diagnosis holds the key to its management. Conventional microbiology has limited utility and nucleic acid-based methods have not been widely accepted for various reasons. In view of the paucibacillary nature of cerebrospinal fluid (CSF) and the recent demonstration of free Mycobacterium tuberculosis DNA in clinical specimens, the present study was designed to evaluate the utility of CSF 'filtrates' for the diagnosis of TBM using PCR. One hundred and sixty-seven CSF samples were analysed from patients with 'suspected' TBM (n=81) and a control group including other cases of meningitis or neurological disorders (n=86). CSF 'sediments' and 'filtrates' were analysed individually for M. tuberculosis DNA by quantitative real-time PCR (qRT-PCR) and conventional PCR. Receiver-operating characteristic curves were generated from qRT-PCR data and cut-off values of 84 and 30 were selected for calling a 'filtrate' or 'sediment' sample positive, respectively. Based on these, TBM was diagnosed with 87.6% and 53.1% sensitivity (P<0.001) in 'filtrates' and 'sediments', respectively, and with 92% specificity each. Conventional devR and IS6110 PCR were also significantly more sensitive in 'filtrates' versus 'sediments' (sensitivity of 87.6% and 85.2% vs 31% and 39.5%, respectively; P<0.001). The qRT-PCR test yielded a positive likelihood ratio of 11 and 6.6 by analysing 'filtrate' and 'sediment' fractions, respectively, which establishes the superiority of the 'filtrate'-based assay over the 'sediment' assay. PCR findings were separately verified in 10 confirmed cases of TBM, where M. tuberculosis DNA was detected using devR PCR assays in 'sediment' and 'filtrate' fractions of all samples. From this study, we conclude that (i) CSF 'filtrates' contain a substantial amount of M. tuberculosis DNA and (ii) 'filtrates' and not 'sediments' are likely to reliably provide a PCR-based diagnosis in 'suspected' TBM patients.

Persistent association of Mycobacterium ulcerans with West African predaceous insects of the family belostomatidae

A number of studies have suggested that Mycobacterium ulcerans, the etiological agent of Buruli ulcer, may be transmitted to humans by insect bites. M. ulcerans has been isolated from a predaceous aquatic insect, and PCR detection of M. ulcerans DNA in aquatic environments suggests that the organism is widely distributed within many invertebrate taxa and functional feeding groups. Thus, M. ulcerans may be concentrated through different trophic links. However, the specific environmental niche of M. ulcerans and route of transmission to humans remain a mystery. In this study, a biologically relevant infection model in which M. ulcerans-infected mosquito larvae were fed to a species of predaceous hemiptera (African Belostomatidae) was used to demonstrate the persistent colonization of M. ulcerans and subsequent transmission of bacteria to naïve prey. The association of M. ulcerans with specific anatomical compartments showed that M. ulcerans accumulates preferentially on the exoskeleton. In contrast, few organisms were found in dissected guts or salivary glands. No difference was found between the ability of wild-type M. ulcerans and an M. ulcerans isogenic mycolactone-negative mutant to colonize belostomatids. These data show that African belostomatids can successfully be colonized by M. ulcerans and support the trophic transfer of M. ulcerans within the environment.

Simplified detection of Mycobacterium tuberculosis in sputum using smear microscopy and PCR with molecular beacons

The prompt diagnosis of smear-negative cases is a prerequisite to controlling tuberculosis (TB). Several new laboratory approaches, including nucleic acid amplification (NAA), are being evaluated in various disease settings to meet this challenge. However, NAA needs simplification before it is widely accepted. Furthermore, a supporting smear result improves confidence in and reliability of PCR. In this context, an asymmetric devR PCR assay using two molecular beacon probes for visual or fluorimetric end-point detection of Mycobacterium tuberculosis was developed. The assays reproducibly detected 25 fg M. tuberculosis DNA versus 100 fg by conventional gel electrophoresis (henceforth referred to as gel assay). The devR and IS6110 PCR assays were blindly evaluated on sputum specimens obtained from a directly observed-treatment short-course centre. Universal sample processing (USP) smear microscopy and culture were used as a supportive test and the 'gold' standard, respectively. Among the 148 specimens analysed, 120 were M. tuberculosis culture-positive. Amongst the 122 direct smear-negative samples, 96 were culture-positive, of which 61 were detected by USP smear microscopy. All 35 USP smear-negative samples were positive by three or more PCR methods. devR PCR had a sensitivity of 92.5 % in the fluorimetric assay versus 86.7 % by visual inspection and 90.8 % by the gel method. IS6110 PCR performed at almost equivalent levels. devR visual and fluorimetric assays considered together yielded an increased sensitivity of 95 % without compromising on a specificity of 92.9 %. The results suggest that the USP smear test is useful for diagnosing direct smear-negative TB and judiciously restricting PCR testing to only smear-negative samples. When used together, these tests can provide rapid diagnosis of smear-negative TB in a cost-effective manner.