Comparison of polymerase chain reaction and standard microbiological techniques in presumed bacterial corneal ulcers

Microbiological culture versus 16S/18S rRNA gene PCR-sanger sequencing for infectious keratitis: a three-arm, diagnostic cross-sectional study

Background

To compare the diagnostic performance of microbiological culture and 16S/18S rRNA gene polymerase chain reaction (PCR)-Sanger sequencing for infectious keratitis (IK) and to analyse the effect of clinical disease severity on test performance and inter-test concordance.

Methods

This was a three-arm, diagnostic cross-sectional study. We included all eligible patients who presented with presumed bacterial/fungal keratitis to the Queen's Medical Centre, Nottingham, UK, between June 2021 and September 2022. All patients underwent simultaneous culture (either direct or indirect culture, or both) and 16S (pan-bacterial)/18S (pan-fungal) ribosomal RNA (rRNA) PCR-Sanger sequencing. The bacterial/fungal genus and species identified on culture were confirmed using matrix-assisted laser desorption/ionization-time-of-flight mass spectrometry. Relevant clinical data were also collected to analyze for any potential clinico-microbiological correlation. Main outcome measures included the diagnostic yield, test accuracy (including sensitivity and specificity), and inter-test agreement [including percent agreement and Cohen's kappa (k)].

Results

A total of 81 patients (86 episodes of IK) were included in this study. All organisms identified were of bacterial origin. Diagnostic yields were similar among direct culture (52.3%), indirect culture (50.8%), and PCR (43.1%; p = 0.13). The addition of PCR enabled a positive diagnostic yield in 3 (9.7%) direct culture-negative cases. Based on composite reference standard, direct culture had the highest sensitivity (87.5%; 95% CI, 72.4-95.3%), followed by indirect culture (85.4%; 95% CI, 71.6-93.5%) and PCR (73.5%; 95% CI, 59.0-84.6%), with 100% specificity noted in all tests. Pairwise comparisons showed substantial agreement among the three tests (percent agreement = 81.8-86.2%, Cohen's k = 0.67-0.72). Clinico-microbiological correlation demonstrated higher culture-PCR concordance in cases with greater infection severity.

Conclusions

This study highlights a similar diagnostic performance of direct culture, indirect culture and 16S rRNA PCR for bacterial keratitis, with substantial inter-test concordance. PCR serves as a useful diagnostic adjuvant to culture, particularly in culture-negative cases or those with lesser disease severity (where culture-PCR concordance is lower).

Rapid Pathogen Detection in Infectious Uveitis Using Nanopore Metagenomic Next-Generation Sequencing: A Preliminary Study

PURPOSE

We establish an accurate and rapid diagnostic method for pathogen detection in infectious uveitis using nanopore metagenomic next-generation sequencing (NGS).

METHODS

In eight patients with suspected infectious uveitis, we prospectively compared the accuracy and time taken for pathogen identification between conventional diagnostic methods, such as cultures and PCR, and nanopore metagenomic NGS.

RESULTS

All causative pathogens were identified using nanopore sequencing, while only five of the eight patients were confirmed positive for a specific pathogen using conventional methods. The overall sample-to-answer turnaround time of nanopore sequencing was much shorter than that of conventional methods in the bacterial and fungal infection (mean, 17 h vs. 4d, respectively; P = .028). The pathogens could be detected even when the quantity or quality of DNA was not optimal.

CONCLUSION

Nanopore metagenomic NGS is a promising diagnostic tool that can rapidly and accurately identify the causative pathogen in infectious uveitis.

Corneal Specialists' Confidence in Identifying Causal Organisms of Microbial Keratitis

PURPOSE

Microbial keratitis (MK) is a potentially blinding corneal disease caused by an array of microbial etiologies. However, the lack of early organism identification is a barrier to optimal care. We investigated clinician confidence in their diagnosis of organism type on initial presentation and the relationship between confidence and presenting features.

METHODS

This research presents secondary data analysis of 72 patients from the Automated Quantitative Ulcer Analysis (AQUA) study. Cornea specialists reported their confidence in organism identification. Presenting sample characteristics were recorded including patient demographics, health history, infection morphology, symptoms, and circumstances of infection. The association between confidence and presenting characteristics was investigated with 2-sample t-tests, Wilcoxon tests, and Chi-square or Fisher's exact tests.

RESULTS

Clinicians reported being "confident or very confident" in their diagnosis of the causal organism in MK infections for 39 patients (54%) and "not confident" for 33 patients (46%). Confidence was not significantly associated with patient demographics, morphologic features, or symptoms related to MK. MK cases where clinicians reported they were confident, versus not confident in their diagnosis, showed significantly smaller percentages of previous corneal disease (0% versus 15%, p = 0.017), were not seen by an outside provider first (69% versus 94%, p = 0.015), or had no prior labs drawn (8% versus 33%, p = 0.046), and a significantly larger percentage of cases wore contact lenses (54% versus 28%, p = 0.029).

CONCLUSION

In almost half of MK cases, cornea specialists reported lack of confidence in identifying the infection type. Confidence was related to ocular history and circumstances of infection but not by observable signs and symptoms or patient demographics. Tools are needed to assist clinicians with early diagnosis of MK infection type to expedite care and healing.

Utility of investigation for suspected microbial keratitis: a diagnostic accuracy study

PURPOSE

The true disease status of a population with suspected microbial keratitis (MK) cannot be verified. There is not an accurate (gold) reference standard to confirm infection and inter-test comparisons of sensitivity and specificity therefore lead to bias with questionable estimates of test utility. We present an alternative method to report results.

METHODS

We used a decision to treat as the definition for MK. We retrospectively compared the results of corneal culture and polymerase chain reaction (PCR) as these are objective tests available for the three principal groups of pathogens. We then estimated the potential contribution of positive results, either alone or in combination, to support the working diagnosis.

RESULTS

We included 2021 (77.4%) eyes with suspected bacterial keratitis, 365 (14.0%) with suspected acanthamoeba keratitis, and 226 (8.6%) with suspected fungal keratitis, all treated between July 2013 and December 2019. In these groups, there were 51.6% positive culture and 6.5% positive PCR results for bacteria, 19.0% and 40.5% for acanthamoeba, and 28.3% and 15.0% for fungi. Between groups the differences in the proportions of positive results from culture and PCR was statistically significant (P < 0.001). The added benefit of PCR to the result of culture in identifying a potential pathogen was 1.4% for bacteria (P = 0.6292), 24.4% for acanthamoeba (P = 0.0001), and 5.8% for fungi (P = 0.3853).

CONCLUSIONS

For suspected MK a comparison of the test positivity rate is an easily comprehensible outcome measure of test utility.

Comparison of culture, confocal microscopy and PCR in routine hospital use for microbial keratitis diagnosis

AIMS

To evaluate the sensitivity and specificity of polymerase chain reaction (PCR), in vivo confocal microscopy (IVCM) and culture for microbial keratitis (MK) diagnosis.

METHODS

Retrospective review of PCR, IVCM and culture results for MK diagnosis at Moorfields Eye Hospital between August 2013 and December 2014.

RESULTS

PCR results were available for 259 MK patients with concurrent culture for 203/259 and IVCM for 149/259. Sensitivities and specificities with 95% confidence intervals [95% CI] were calculated for Acanthamoeba keratitis (AK) and fungal keratitis (FK), by comparison with culture, for both IVCM and PCR. For AK, FK and bacterial keratitis (BK) sensitivities were calculated, for each diagnostic method, by comparison with a composite reference standard (a positive result for one or more of culture, PCR or IVCM having a specificity of 100% by definition). For the latter, sensitivities with [95% CI] were: for AK, IVCM 77.1% [62.7-88.0%], PCR 63.3% [48.3-76.6%], culture 35.6 [21.9-51.2]; for FK, IVCM 81.8% [48.2-97.7%], PCR 30.8% [9.09-61.4%], culture 41.7% [15.2-72.3%]; for BK, PCR 25.0% [14.7-37.9%], culture 95.6% [87.6-99.1%].

CONCLUSION

IVCM was the most sensitive technique for AK and FK diagnosis but culture remains our gold standard for BK. These findings reflect results to be expected from service providers to UK ophthalmology units and demonstrates the need at our centre for ongoing diagnostic result audit leading to the potential to improve PCR diagnosis. Both FK and AK are now common in the UK; ophthalmology units need to have all these techniques available to optimise their MK management.

Diagnostic methods for the etiological assessment of infectious corneal pathology (Review)

Infectious keratitis is a leading cause of visual morbidity, including blindness, all across the globe, especially in developing countries. Prompt and adequate treatment is mandatory to maintain corneal integrity and to recover the best possible final visual acuity. Although in most of the cases practitioners chose to employ empirical broad-spectrum antimicrobial medication that is usually effective, in some instances, they face the need to identify the causative agent to establish the appropriate therapy. An extensive search was conducted on published literature before December 2020 concerning the main laboratory investigations used to identify the microbial agents found in infectious keratitis, their indications, advantages, and disadvantages, as well as the results reported by other studies concerning different diagnostic tools. At present, the gold standard for diagnosis is still considered to be the isolation of microorganisms in cultures, along with the examination of smears, but other newer techniques, such as polymerase chain reaction (PCR), next-generation sequencing (NGS), and in vivo confocal microscopy (IVCM) have gained popularity in the last decades. Currently, these newer methods have proved to be valuable adjuvants in making the diagnosis, but technological advances hold promise that, in the future, these methods will have increased performance and availability, and may become the new gold standard, replacing the classic cultures and smears.

Diagnostic armamentarium of infectious keratitis: A comprehensive review

Infectious keratitis (IK) represents the leading cause of corneal blindness worldwide, particularly in developing countries. A good outcome of IK is contingent upon timely and accurate diagnosis followed by appropriate interventions. Currently, IK is primarily diagnosed on clinical grounds supplemented by microbiological investigations such as microscopic examination with stains, and culture and sensitivity testing. Although this is the most widely accepted practice adopted in most regions, such an approach is challenged by several factors, including indistinguishable clinical features shared among different causative organisms, polymicrobial infection, long diagnostic turnaround time, and variably low culture positivity rate. In this review, we aim to provide a comprehensive overview of the current diagnostic armamentarium of IK, encompassing conventional microbiological investigations, molecular diagnostics (including polymerase chain reaction and mass spectrometry), and imaging modalities (including anterior segment optical coherence tomography and in vivo confocal microscopy). We also highlight the potential roles of emerging technologies such as next-generation sequencing, artificial intelligence-assisted platforms. and tele-medicine in shaping the future diagnostic landscape of IK.

16S Ribosomal RNA PCR Versus Conventional Diagnostic Culture in the Investigation of Suspected Bacterial Keratitis

Purpose

The purpose of this study was to compare conventional diagnostic culture (CDC) to 16S ribosomal RNA polymerase chain reaction (PCR) analysis for diagnosing bacterial keratitis.

Methods

Samples collected from 100 consecutive patients presenting to the Royal Liverpool University Hospital with bacterial keratitis were processed using CDC and 16S PCR analysis.

Results

The overall detection rate using both methods was 36%. Of these, 72.2% (26/36) were detected by PCR and 63.9% (23/36) isolated by CDC (P = 0.62). Using a combination of both PCR and CDC increased the detection rate for pathogenic bacteria by 13% compared to using CDC alone (P = 0.04). In CDC negative samples, 16S PCR identified more pathogens than CDC in 16S PCR negative samples. Neither order of sample collection nor prior antimicrobial use affected the detection rate.

Conclusions

16S rRNA gene PCR performed in addition to CDC on corneal samples from patients with clinically suspected bacterial keratitis led to additional pathogen detection.

Translational Relevance

16S rRNA gene PCR should be developed to become an additional part of clinical service for patients with bacterial keratitis rather than used in isolation.

Effectiveness of 16S ribosomal DNA real-time PCR and sequencing for diagnosing bacterial keratitis

PURPOSE

To evaluate the efficacy of real-time PCR for 16S ribosomal DNA (16S r-DNA) and sequencing for diagnosing microbial keratitis.

METHODS

We studied 272 eyes of 272 patients with keratitis. Eyes with keratitis were classified as "definite" (N = 118), "likely" (N = 71), or "non-bacterial" (N = 83) to have bacterial keratitis. The diagnostic efficacy of real-time PCR and conventional testing was determined by receiver operating characteristic analysis. The copy numbers of bacterial DNA and clinical characteristics were retrospectively analyzed for association with concordant culture results in the "definite" cases.

RESULTS

The level of bacterial DNA was significantly associated with the diagnostic probability of the three diagnostic categories. The level of bacterial DNA had comparable diagnostic efficacy with the area under the curve (AUC) at 0.67, by culture at 0.65, and by smear testing at 0.73. The efficacy was significantly improved by combining the DNA level with the conventional culture testing with an AUC of 0.81. Analysis of the "definite" cases showed culture positivity in 51.8% (58 eyes), and of these, 41 eyes (70.7%) were higher than the cutoff PCR values and 40 eyes were identified by 16S r-DNA sequencing. In the culture-negative eyes, the level of bacterial DNA was significantly lower (P = 0.0008). Eyes with higher bacterial DNA levels had significantly concordant outcomes with sequencing and culture results (P = 0.006). Previous antibiotic treatments decreased the bacterial DNA amount by 0.09-fold, and it was a significant factor for discordance (P = 0.006).

CONCLUSION

Quantification of the bacterial DNA level and conventional testing improves the diagnostic efficacy of infectious bacterial keratitis.

Evolving nanotechnological trends in the management of mycotic keratitis

The alarming increase in global burden of various corneal diseases in general and mycotic keratitis in particular has raised up a major concern for its treatment. Mycotic keratitis is one of the most serious infections among the various ocular diseases. The proper diagnosis and effective treatment strategies remain a great challenge for ophthalmologists. The inefficacy and failure of conventional treatments have generated need to develop alternative approaches for the treatment of mycotic keratitis. Considering the promising applications of nanotechnology in biomedical area, it is believed that various nanomaterials can be effectively used in the management of mycotic keratitis. This review focuses on worldwide burden of various corneal diseases including mycotic keratitis and the role of nanotechnology in its treatment. In addition, safety and toxicological issues are also discussed.