Rapid detection and typing of pathogenic nonpneumophila Legionella spp. isolates using a multiplex real-time PCR assay

A rapid and reliable method for early Legionella pneumophila identification and characterization in support of the epidemiology study

Introduction

Legionnaires' disease is a severe pneumonia predominantly caused by Legionella pneumophila (Lp), whose major reservoirs are artificial water systems. As most human infections are caused by L. pneumophila serogroup 1 (Lp1), a reliable method for Lp distinction can be crucial for bacterial spread prevention. As the ability to withstand in environments and to cause the waterborne disease is strongly related to specific genes, the identification of virulent strains can be of great relevance to implement water environmental monitoring and to contain harmful outbreaks to public health. We aimed to test an assay for Lp identification among different Legionella species, and to determine the serogroups. Additionally, we investigated the carriage of virulence and antimicrobial resistance genes.

Methods

A total of 90 Legionella spp. isolates identified by phenotypic tests were subjected to the designed quantitative PCR assay targeting specific mip for Lp, wzm for Lp1, pvcA and ahpD for biofilm production. Eleven serogroups were investigated in all our isolates tested positive for mip gene, subsequently analyzed for 12 virulence and 8 antimicrobial resistance genes.

Results

Only the 70 Lp isolates were positive for mip. Out of 27 Lp isolates belonging to serogroup 1 based on agglutination test, 23 (85.2%) carried wzm. The presence of ahpD and pvcA was found in 94.3 and 98.6% of Lp isolates, respectively. By multiplex PCR, all 23 wzm-positive strains were confirmed as serogroup 1 that was the most predominant (33%). At least one virulence gene was detected in all Lp isolates. The most frequent gene was ispE (100%), followed by issD (96%), icmK and enhC (93%), cpxA (91%), rtxA2 (74%), lvhB8-B9 (61%), and prpA (54%). The other genes were less diffused in Lp strains (rtxA1, 44%; lvhB3-B4, 47%; pvcB, 27%; lvrE, 24%). Of the macrolide resistance genes, the ereA was found in 84% of Lp strains, while only 14 (20%) harbored the lpeAB among the efflux pump genes.

Conclusion

The assays validated in this study enable the simultaneous Lp and Lp1 detection. The differentiation of Lp strains according to their virulence properties could be useful to predict the bacterial ability to survive and to cause the disease.

Overview of the Clinical and Molecular Features of Legionella Pneumophila: Focus on Novel Surveillance and Diagnostic Strategies

Legionella pneumophila (L. pneumophila) is one of the most threatening nosocomial pathogens. The implementation of novel and more effective surveillance and diagnostic strategies is mandatory to prevent the occurrence of legionellosis outbreaks in hospital environments. On these bases, the present review is aimed to describe the main clinical and molecular features of L. pneumophila focusing attention on the latest findings on drug resistance mechanisms. In addition, a detailed description of the current guidelines for the disinfection and surveillance of the water systems is also provided. Finally, the diagnostic strategies available for the detection of Legionella spp. were critically reviewed, paying the attention to the description of the culture, serological and molecular methods as well as on the novel high-sensitive nucleic acid amplification systems, such as droplet digital PCR.

Legionella SBT applied directly to respiratory samples as a rapid molecular epidemiological tool

Legionnaires' disease (LD) is an atypical pneumonia caused by the inhalation of Legionella. The methods used for the diagnosis of LD are direct culture of respiratory samples and urinary antigen detection. However, the sensitivity of culture is low, and the urinary antigen test is specific only for L. pneumophila sg1. Moreover, as no isolates are obtained, epidemiological studies cannot be performed. The implementation of Nested-sequence-based typing (Nested-SBT) makes it possible to carry out epidemiological studies while also confirming LD, especially in cases caused by non-sg 1. Sixty-two respiratory samples from patients with Legionella clinically confirmed by positive urinary antigen tests were cultured and tested by Nested-SBT, following the European Study Group for Legionella Infections (ESGLI) protocol. Only 2/62 (3.2%) respiratory samples were culture-positive. Amplification and sequencing of Nested-SBT genes were successfully performed in 57/62 samples (91.9%). The seven target genes were characterised in 39/57 (68.4%) respiratory samples, and the complete sequence type (ST) was obtained. The mip gene was the most frequently amplified and sequenced. Nested-SBT is a useful method for epidemiological studies in culture-negative samples, achieving a 28.7-fold improvement over the results of culture studies and reducing the time needed to obtain molecular epidemiological results.

Simultaneous detection of Legionella species and L. anisa, L. bozemanii, L. longbeachae and L. micdadei using conserved primers and multiple probes in a multiplex real-time PCR assay

Legionnaires' disease is a severe respiratory disease that is estimated to cause between 8,000 and 18,000 hospitalizations each year, though the exact burden is unknown due to under-utilization of diagnostic testing. Although Legionella pneumophila is the most common species detected in clinical cases (80-90%), other species have also been reported to cause disease. However, little is known about Legionnaires' disease caused by these non-pneumophila species. We designed a multiplex real-time PCR assay for detection of all Legionella spp. and simultaneous specific identification of four clinically-relevant Legionella species, L. anisa, L. bozemanii, L. longbeachae, and L. micdadei, using 5'-hydrolysis probe real-time PCR. The analytical sensitivity for detection of nucleic acid from each target species was ≤50fg per reaction. We demonstrated the utility of this assay in spiked human sputum specimens. This assay could serve as a tool for understanding the scope and impact of non-pneumophila Legionella species in human disease.