Legionnaires' disease caused by Legionella longbeachae: Clinical features and outcomes of 107 cases from an endemic area

Hairy Cell Leukemia (HCL) Presenting As Severe Acute Respiratory Distress Syndrome (ARDS) With Legionella pneumophila: Coincidence or Causation?

Due to a low index of suspicion coupled with specific growth conditions and non-specific clinical manifestations, Legionella (L.) pneumophila is a frequently misdiagnosed cause of pneumonia in immunocompromised patients, especially those with hematological malignancies. We present a case of severe acute respiratory distress syndrome (ARDS) secondary to Legionnaire's disease in a patient with newly diagnosed hairy cell leukemia (HCL) to highlight the importance of early recognition, diagnosis, and treatment of Legionnaire's disease to reduce morbidity and mortality.

Eat or Be Eaten: Strategies Used by Legionella to Acquire Host-Derived Nutrients and Evade Lysosomal Degradation

To replicate within host cells, bacterial pathogens must acquire host-derived nutrients while avoiding degradative antimicrobial pathways. Fundamental insights into bacterial pathogenicity have been revealed by bacteria of the genus Legionella, which naturally parasitize free-living protozoa by establishing a membrane-bound replicative niche termed the Legionella-containing vacuole (LCV). Biogenesis of the LCV and intracellular replication rely on rapid evasion of the endocytic pathway and acquisition of host-derived nutrients, much of which is mediated by bacterial effector proteins translocated into host cells by a Dot/Icm type IV secretion system. Billions of years of co-evolution with eukaryotic hosts and broad host tropism have resulted in expansion of the Legionella genome to accommodate a massive repertoire of effector proteins that promote LCV biogenesis, safeguard the LCV from endolysosomal maturation, and mediate the acquisition of host nutrients. This minireview is focused on the mechanisms by which an ancient intracellular pathogen leverages effector proteins and hijacks host cell biology to obtain essential host-derived nutrients and prevent lysosomal degradation.

Clinical and Laboratory Diagnosis of Legionella Pneumonia

Legionella pneumonia is a relatively rare but extremely progressive pulmonary infection with high mortality. Traditional cultural isolation remains the gold standard for the diagnosis of Legionella pneumonia. However, its harsh culture conditions, long turnaround time, and suboptimal sensitivity do not meet the clinical need for rapid and accurate diagnosis, especially for critically ill patients. So far, pathogenic detection techniques including serological assays, urinary antigen tests, and mass spectrometry, as well as nucleic acid amplification technique, have been developed, and each has its own advantages and limitations. This review summarizes the clinical characteristics and imaging findings of Legionella pneumonia, then discusses the advances, advantages, and limitations of the various pathogenetic detection techniques used for Legionella pneumonia diagnosis. The aim is to provide rapid and accurate guiding options for early identification and diagnosis of Legionella pneumonia in clinical practice, further easing healthcare burden.

Presence of Viable, Clinically Relevant Legionella Bacteria in Environmental Water and Soil Sources of China

The distribution of pathogenic Legionella in the environmental soil and water of China has not been documented yet. In this study, Legionella was detected in 129 of 575 water (22.43%) and 41 of 442 soil samples (9.28%) by culture. Twelve Legionella species were identified, of which 11 were disease-associated. Of the Legionella-positive samples, 109 of 129 (84.50%) water and 29 of 41 (70.73%) soil were positive for L. pneumophila, which accounted for about 75% of Legionella isolates in both water and soil, suggesting L. pneumophila was the most frequent species. Soil showed a higher diversity of Legionella spp. as compared with water (0.6279 versus 0.4493). In contrast, serogroup (sg) 1 was more prevalent among L. pneumophila isolates from water than from soil (26.66% versus 12.21%). Moreover, many disease-associated sequence types (STs) of L. pneumophila were found in China. Intragenic recombination was acting on L. pneumophila from both water and soil. Phylogeny, population structure, and molecular evolution analyses revealed a probable existence of L. pneumophila isolates with a special genetic background that is more adaptable to soil or water sources and a small proportion of genetic difference between water and soil isolates. The detection of viable, clinically relevant Legionella demonstrates soil as another source for harboring and dissemination of pathogenic Legionella bacteria in China. Future research should assess the implication in public health with the presence of Legionella in the soil and illustrate the genetic and pathogenicity difference of Legionella between water and soil, particularly the most prevalent L. pneumophila. IMPORTANCE Pathogenic Legionella spp. is the causative agent of Legionnaires' disease (LD), and L. pneumophila is the most common one. Most studies have focused on L. pneumophila from water and clinical samples. However, the soil is another important reservoir for this bacterium, and the distribution of Legionella spp. in water and soil sources has not been compared and documented in China yet. Discovering the distribution of Legionella spp. and L. pneumophila in the two environments may help a deep understanding of the pathogenesis and molecular evolution of the bacterium. Our research systematically uncovered the distributions of Legionella spp. in different regions and sources (e.g., water and soil) of China. Moreover, phylogeny, population structure, and molecular evolution study revealed the possible existence of L. pneumophila with a special genetic background that is more adaptable to soil or water sources, and genetic difference may exist.

Validating a clinical prediction score for Legionella-related community acquired pneumonia

BACKGROUND

Legionella-related community acquired pneumonia (CAP) is a disease with an increasing incidence and a high mortality rate, especially if empirical antibiotic therapy is inadequate. Antibiotic treatment highly relies on clinical symptoms, although proven non-specific, because currently available diagnostic techniques provide insufficient accuracy for detecting Legionella CAP on admission. This study validates a diagnostic scoring system for detection of Legionella-related CAP, based on six items on admission (Legionella prediction score).

METHODS

We included patients with Legionella-related CAP admitted to five large Dutch hospitals between 2006 and 2016. Controls were non-Legionella-related CAP patients. The following six conditions were rewarded one point if present: fever > 39.4 °C; dry cough; hyponatremia (sodium) < 133 mmol/L; lactate dehydrogenase (LDH) > 225 mmol/L; C-reactive protein (CRP) > 187 mg/L and platelet count < 171 × 10⁹/L. The accuracy of the prediction score was assessed by calculating the area under the curve (AUC) through logistic regression analysis.

RESULTS

We included 131 cases and 160 controls. A score of 0 occurred in non-Legionella-related CAP patients only, a score of 5 and 6 in Legionella-related CAP patients only. A cut-off ≥ 4 resulted in a sensitivity of 58.8% and a specificity of 93.1%. The AUC was 0.89 (95% CI 0.86-0.93). The strongest predictors were elevated LDH, elevated CRP and hyponatremia.

CONCLUSIONS

This multi-centre study validates the Legionella prediction score, an easily applicable diagnostic scoring system, in a large group of patients and finds high diagnostic accuracy. The score shows promise for future prospective validation and could contribute to targeted antibiotic treatment of suspected Legionella CAP.

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.

Diagnostic accuracy of urinary antigen tests for legionellosis: A systematic review and meta-analysis

BACKGROUND

Urinary antigen tests (UATs) have been used for the early detection of legionellosis and have demonstrated moderate sensitivity and high specificity. However, the most recent systematic review and meta-analysis published in 2009 evaluated the accuracy of UATs; since then, UAT accuracy may have changed owing to advances and developments in UAT technology and epidemiological changes in the frequency of Legionella species that cause legionellosis. Therefore, this systematic review and meta-analysis aimed to update the accuracy of UATs for legionellosis among patients with suspected pneumonia.

METHODS

Overall, 1326 studies were screened, 21 of which fulfilled the eligibility criteria for quality assessment and meta-analysis. Data from 5772 patients, including 1368 (23.7%) with the target condition (i.e., suspected legionellosis), were included in the analysis. The overall quality of the included studies, which was assessed using the Quality Assessment of Diagnostic Accuracy Studies-2 tool, was unclear.

RESULTS

The calculated pooled sensitivity and specificity were 0.79 (95% confidence interval [CI], 0.71-0.85) and 1.00 (95% CI, 0.99-1.00), respectively. Subpopulation analysis revealed that the accuracy of UATs for sensitivity and specificity for Legionella pneumophilia serogroup 1 was 0.86 (95% CI, 0.78-0.91) and 1.00 (95% CI, 0.99-1.00), respectively.

CONCLUSIONS

This study demonstrated that the sensitivity and specificity of UATs were moderate and high, respectively, which is comparable to the results reported in 2009. Therefore, UATs may be a useful method for the early detection of legionellosis caused by Legionella pneumophila serogroup 1.

CLINICAL TRIAL REGISTRATION

The review protocol was prospectively registered with the University Hospital Medical Information Network Clinical Trials Registry (UMIN000041080).

Complement-mediated autoimmune haemolytic anaemia as an initial presentation of Legionnaires' disease

A 42-year-old diabetic man presented to the hospital with severe sepsis and multiorgan dysfunction. A probable respiratory source of sepsis was suspected because of suggestive clinical and radiological findings. He was critically ill and was therefore admitted to intensive care for further management including ventilatory support and renal replacement therapy. He was also found to have marked anaemia requiring multiple blood transfusions with clinical and laboratory evidence pointing towards severe haemolysis. Further workup for the aetiology of pneumonia established a diagnosis of Legionella by confirmatory tests namely legionella antigen in the urine and exponentially rising serum antibody titres. The cause for the severe haemolysis was found to be complement-mediated autoimmune haemolysis as determined by direct antiglobulin test positive for complement components C3 and negative for IgG. Such clinically significant autoimmune haemolysis as a presenting feature, rather than a late complication, has never before been reported in the literature.

How safe are gloves and masks used for protection against Legionella longbeachae infection when gardening?

Legionella longbeachae has been frequently identified in composted plant material and can cause Legionnaires' disease (LD). We wanted to determine how frequently L. longbeachae DNA was present on gardeners' gloves, and how long L. longbeachae could persist on inoculated gloves and masks. Volunteers completed a survey of gardening practices and their gardening gloves were tested for L. longbeachae DNA by qPCR. The persistence of viable L. longbeachae was assessed by timed subcultures after inoculation of gardening gloves and masks. Gloves but not masks were used regularly. L. longbeachae was detected on 11 (14%; 95% CI 8-24%) gloves. Viable organisms were recovered from 25-50% of inoculated cotton, leather and PU coated gloves but not rubber gloves after 8 h incubation. There was a difference in dose-response curve slopes by glove material (P = 0·001) and time to 50% sterility (P = 0·036). There were differences in persistence of L. longbeachae between mask types from analysis of the slopes and 50% sterility on the decay curves (P = 0·042, P < 0·001 respectively). Gardening gloves and masks may act as a vector for transmission of L. longbeachae during gardening. Washing gardening gloves and prompt disposal of masks could reduce risk of LD.

Epileptic Seizure after Use of Moxifloxacin in Man with Legionella longbeachae Pneumonia

Legionellosis caused by Legionella longbeachae is diagnosed mainly by PCR. We report a case of L. longbeachae infection in mainland China, which was diagnosed by metagenomic next-generation sequencing, in a man who developed an epileptic seizure after using moxifloxacin. Metagenomic next-generation sequencing may be a useful tool to detect Legionella spp.

Travel Trouble With Legionella in the Era of COVID-19: A Case Report

A 56-year-old male was admitted to the hospital with severe sepsis secondary to pneumonia. His presentation was challenging and confusing due to the accompanying coronavirus disease 2019 (COVID-19) infection attributed to his travel history and diagnosed via radiological findings. He received dexamethasone with ceftriaxone and azithromycin. Despite the fact he was on appropriate antibiotics, his condition worsened, and he was eventually diagnosed with Legionella pneumonia, which was thought to be resistant to macrolides. His condition improved significantly when antibiotics were switched to levofloxacin. It is important to keep in mind other causes of community-acquired pneumonia (CAP) during the ongoing COVID-19 era. What makes this case unique is that it presented a confusing scenario due to the patient's concurrent COVID-19 infection and his failure to improve with the administration of azithromycin.

Legionellosis Caused by Non-Legionella pneumophila Species, with a Focus on Legionella longbeachae

Although known as causes of community-acquired pneumonia and Pontiac fever, the global burden of infection caused by Legionella species other than Legionella pneumophila is under-recognised. Non-L. pneumophila legionellae have a worldwide distribution, although common testing strategies for legionellosis favour detection of L. pneumophila over other Legionella species, leading to an inherent diagnostic bias and under-detection of cases. When systematically tested for in Australia and New Zealand, L. longbeachae was shown to be a leading cause of community-acquired pneumonia. Exposure to potting soils and compost is a particular risk for infection from L. longbeachae, and L. longbeachae may be better adapted to soil and composting plant material than other Legionella species. It is possible that the high rate of L. longbeachae reported in Australia and New Zealand is related to the composition of commercial potting soils which, unlike European products, contain pine bark and sawdust. Genetic studies have demonstrated that the Legionella genomes are highly plastic, with areas of the chromosome showing high levels of recombination as well as horizontal gene transfer both within and between species via plasmids. This, combined with various secretion systems and extensive effector repertoires that enable the bacterium to hijack host cell functions and resources, is instrumental in shaping its pathogenesis, survival and growth. Prevention of legionellosis is hampered by surveillance systems that are compromised by ascertainment bias, which limits commitment to an effective public health response. Current prevention strategies in Australia and New Zealand are directed at individual gardeners who use potting soils and compost. This consists of advice to avoid aerosols generated by the use of potting soils and use masks and gloves, but there is little evidence that this is effective. There is a need to better understand the epidemiology of L. longbeachae and other Legionella species in order to develop effective treatment and preventative strategies globally.

Legionella longbeachae pneumonia: A case report and literature review in Japan

Herein, we report the case of a 74-year-old man diagnosed with Legionella pneumonia detected by Loop-Mediated Isothermal Amplification (LAMP) method, which was suspected to have been transmitted from the potting soil. Legionella longbeachae was identified in the sputum culture. The patient was intubated and maintained on mechanical ventilation. Antimicrobial therapy with azithromycin was also administered. His symptoms were resolved and he was discharged after 26 days of hospitalization. Legionella longbeachae pneumonia rarely occurs in Japan, and published literature of Legionella longbeachae pneumonia cases in Japan was reviewed. Patients with severe pneumonia exposed to potting soils, but with negative urinary antigen test results, should be examined by LAMP method.

Enhancement of Culture of Legionella longbeachae from Respiratory Samples by Use of Immunomagnetic Separation and Antimicrobial Decontamination

Legionella longbeachae is the commonest Legionella species identified in patients with community-acquired pneumonia in New Zealand. Isolation of the organism on culture is the gold standard for the diagnosis of Legionnaires disease, but it has poor sensitivity (40%) compared with quantitative PCR (qPCR). We have developed a selective decontamination process using glycine, vancomycin, polymyxin, and cycloheximide (GVPC) with immunomagnetic separation (IMS) for culturing L. longbeachae A polyclonal antibody specific for L. longbeachae was produced from New Zealand White rabbits and coupled to tosyl-activated magnetic beads. Stored L. longbeachae qPCR-positive respiratory samples were retrieved from -80°C storage for testing. One portion of test samples was mixed with GVPC and the antibody bead complex, separated, washed, and cultured on modified Wadowsky and Yee agar (MWY) agar. Another portion was exposed to HCl-KCl acidic buffer (pH 2.2) before incubation on MWY agar. qPCR used probes specific for the ITS (internal transcribed spacer) region of the L. longbeachae genome. Cultures were positive in 10/53 (19%) samples after acid wash and 26/53 (49%) after GVPC-IMS (P = 0.001). Growth of contaminants was rare. The mean qPCR threshold cycle values were lower in culture-positive samples after acid wash than in the culture-negative samples (mean, 29.9 versus 34.8; difference, 4.9; 95% confidence interval [CI], ±2.9; P = 0.001) but not after GVPC-IMS (mean, 33.0 versus 34.7; difference, 1.7; 95% CI, ±2.48; P = 0.16). The sensitivity of culture for L. longbeachae in respiratory specimens may be improved by using GVPC-IMS rather than acid wash for decontamination, but this should be confirmed in a prospective study of fresh specimens.

Persistent Legionella contamination of water faucets in a tertiary hospital in Japan

OBJECTIVE

The feasibility of the decontamination procedure for Legionella pneumophila of water systems in healthcare facilities varies by water purification and disinfection methods in each country. We evaluated the efficacy of feasible decontamination strategies in Japan.

METHODS

This study was conducted at Tokyo Medical University Hospital (1015 beds) between 2015 and 2018. Samples from the water system and cooling tower were cultured periodically. Hyper-chlorination of cool tap water (>0.2 ppm), increases in the temperature of hot water (>55 °C), and flushing were used as decontamination strategies. The case of healthcare-associated legionellosis was surveyed. Environmental and clinical isolates were genotyped.

RESULTS

1439 environmental samples were collected; 19 (1.3%) samples tested positive for L. pneumophila from water faucets of patient rooms, toilets, waste rooms, and water sourced from wells. Genotyping of 12 isolates confirmed that the same strains were present in eight environmental isolates and two isolates from patients over three years. Although the environmental contamination of the water system was persistent, the number of positive locations of hospital environments gradually decreased; eight in 2015, four in 2016, three in 2017, and four in 2018, respectively.

CONCLUSIONS

Monitoring contamination, hyper-chlorination, controlling temperature, and flushing were effective as a Legionella decontamination strategy.

In vitro and intracellular activities of frog skin temporins against Legionella pneumophila and its eukaryotic hosts

Temporin-SHa (SHa) is a small cationic host defence peptide (HDP) produced in skin secretions of the Sahara frog Pelophylax saharicus. This peptide has a broad-spectrum activity, efficiently targeting bacteria, parasites and viruses. Noticeably, SHa has demonstrated an ability to kill Leishmania infantum parasites (amastigotes) within macrophages. Recently, an analog of SHa with an increased net positive charge, named [K³]SHa, has been designed to improve those activities. SHa and [K³]SHa were both shown to exhibit leishmanicidal activity mainly by permeabilization of cell membranes but could also induce apoptotis-like death. Temporins are usually poorly active against Gram-negative bacteria whereas many of these species are of public health interest. Among them, Legionella pneumophila, the etiological agent of Legionnaire’s disease, is of major concern. Indeed, this bacterium adopts an intracellular lifestyle and replicate inside alveolar macrophages likewise inside its numerous protozoan hosts. Despite several authors have studied the antimicrobial activity of many compounds on L. pneumophila released from host cells, nothing is known about activity on intracellular L. pneumophila within their hosts, and subsequently mechanisms of action that could be involved. Here, we showed for the first time that SHa and [K³]SHa were active towards several species of Legionella. Both peptides displayed bactericidal activity and caused a loss of the bacterial envelope integrity leading to a rapid drop in cell viability. Regarding amoebae and THP-1-derived macrophages, SHa was less toxic than [K³]SHa and exhibited low half maximal lethal concentrations (LC₅₀). When used at non-toxic concentration (6.25 µM), SHa killed more than 90% L. pneumophila within amoebae and around 50% within macrophages. Using SHa labeled with the fluorescent dye Cy5, we showed an evenly diffusion within cells except in vacuoles. Moreover, SHa was able to enter the nucleus of amoebae and accumulate in the nucleolus. This subcellular localization seemed specific as macrophages nucleoli remained unlabeled. Finally, no modifications in the expression of cytokines and HDPs were recorded when macrophages were treated with 6.25 µM SHa. By combining all data, we showed that temporin-SHa decreases the intracellular L. pneumophila load within amoebae and macrophages without being toxic for eukaryotic cells. This peptide was also able to reach the nucleolus of amoebae but was not capable to penetrate inside vacuoles. These data are in favor of an indirect action of SHa towards intracellular Legionella and make this peptide a promising template for further developments.

Risk Factors for Legionella longbeachae Legionnaires' Disease, New Zealand

Gardeners with chronic obstructive pulmonary disease or a history of smoking should take extra precautions to prevent infection.

Legionella longbeachae, found in soil and compost-derived products, is a globally underdiagnosed cause of Legionnaires’ disease. We conducted a case–control study of L. longbeachae Legionnaires’ disease in Canterbury, New Zealand. Case-patients were persons hospitalized with L. longbeachae pneumonia, and controls were persons randomly sampled from the electoral roll for the area served by the participating hospital. Among 31 cases and 172 controls, risk factors for Legionnaires’ disease were chronic obstructive pulmonary disease, history of smoking >10 years, and exposure to compost or potting mix. Gardening behaviors associated with L. longbeachae disease included having unwashed hands near the face after exposure to or tipping and troweling compost or potting mix. Mask or glove use was not protective among persons exposed to compost-derived products. Precautions against inhaling compost and attention to hand hygiene might effectively prevent L. longbeachae disease. Long-term smokers and those with chronic obstructive pulmonary disease should be particularly careful.

Combining Environmental Investigation and a Dual-Analytical Strategy to Isolate the Legionella longbeachae Strain Linked to Two Occupational Cases of Legionellosis

Legionella has a global distribution, mainly in aquatic and man-made environments. Under the right conditions, this bacterium is a notorious human pathogen responsible for severe pulmonary illnesses. Legionellosis outbreaks are reported around the world, and exposure to water droplet aerosols containing Legionella pneumophila is usually the mechanism of its transmission. Even if L. pneumophila causes most outbreaks, Legionella longbeachae also accounts for some cases. Unlike most other Legionella strains, L. longbeachae is typically found in soil. Given the wide diversity and high concentration of microorganisms found in soil, isolating L. longbeachae by culture can be challenging. Because the chances of successfully isolating the strain are low, it is often not even attempted. This study reports the strategies used to successfully isolate L. longbeachae strain that was responsible of the two occupational legionellosis in Quebec. Fifteen random samples were collected from the soil of the metal recycling plant where the diagnosed workers were employed, covering 1.5% of the accessible surface of the plant. All samples were analyzed with both the quantitative polymerase chain reaction (qPCR) and culture methods. Four qPCR detection systems targeting Legionella spp, L. pneumophila, L. pneumophila serogroup 1, and L. longbeachae were used. Acid, heat, and acid/heat treatments were used for the culture method. For the qPCR method, all samples were positives for Legionella spp but only four were positives for L. longbeachae. For the culture method, only one isolate could be confirmed to be L. longbeachae. However, that strain proves to be the same one that caused the occupational legionellosis. Detecting the presence of L. longbeachae using the qPCR method made it possible to target the right samples to enable the cultivable strain of L. longbeachae to be isolated from the soil of the metal recycling plant. The complementarity of the two methods was established. This paper demonstrated the advantages of selecting the proper sampling and analytical strategies to achieve the isolation of the strain responsible for the infections. It also highlights for the first time in Quebec the potential occupational risks associated with L. longbeachae from soil and should motivate questioning soil exposures when all sources of water contamination have been eliminated from the causal analysis of legionellosis.

Ten Questions Concerning the Aerosolization and Transmission of Legionella in the Built Environment

Legionella is a genus of pathogenic Gram-negative bacteria responsible for a serious disease known as legionellosis, which is transmitted via inhalation of this pathogen in aerosol form. There are two forms of legionellosis: Legionnaires' disease, which causes pneumonia-like symptoms, and Pontiac fever, which causes influenza-like symptoms. Legionella can be aerosolized from various water sources in the built environment including showers, faucets, hot tubs/swimming pools, cooling towers, and fountains. Incidence of the disease is higher in the summertime, possibly because of increased use of cooling towers for air conditioning systems and differences in water chemistry when outdoor temperatures are higher. Although there have been decades of research related to Legionella transmission, many knowledge gaps remain. While conventional wisdom suggests that showering is an important source of exposure in buildings, existing measurements do not provide strong support for this idea. There has been limited research on the potential for Legionella transmission through heating, ventilation, and air conditioning (HVAC) systems. Epidemiological data suggest a large proportion of legionellosis cases go unreported, as most people who are infected do not seek medical attention. Additionally, controlled laboratory studies examining water-to-air transfer and source tracking are still needed. Herein, we discuss ten questions that spotlight current knowledge about Legionella transmission in the built environment, engineering controls that might prevent future disease outbreaks, and future research that is needed to advance understanding of transmission and control of legionellosis.

Seeing red; the development of pON.mCherry, a broad-host range constitutive expression plasmid for Gram-negative bacteria

The development of plasmid-mediated gene expression control in bacteria revolutionized the field of bacteriology. Many of these expression control systems rely on the addition of small molecules, generally metabolites or non-metabolized analogs thereof, to the growth medium to induce expression of the genes of interest. The paradigmatic example of an expression control system is the lac system from Escherichia coli, which typically relies on the Ptac promoter and the Lac repressor, LacI. In many cases, however, constitutive gene expression is desired, and other experimental approaches require the coordinated control of multiple genes. While multiple systems have been developed for use in E. coli and its close relatives, the utility and/or functionality of these tools does not always translate to other species. For example, for the Gram-negative pathogen, Legionella pneumophila, a causative agent of Legionnaires' Disease, the aforementioned Ptac system represents the only well-established expression control system. In order to enhance the tools available to study bacterial gene expression in L. pneumophila, we developed a plasmid, pON.mCherry, which confers constitutive gene expression from a mutagenized LacI binding site. We demonstrate that pON.mCherry neither interferes with other plasmids harboring an intact LacI-Ptac expression system nor alters the growth of Legionella species during intracellular growth. Furthermore, the broad-host range plasmid backbone of pON.mCherry allows constitutive gene expression in a wide variety of Gram-negative bacterial species, making pON.mCherry a useful tool for the greater research community.