1 INTRODUCTION

The recent Listeria outbreak reports in the United States are concerning. It is worrying primarily for two reasons. One, aerobic bacterial food infections and intoxication cases are quite rare owing to prevailing typical anaerobic food packaging practices there, and two, the US food and pharmaceutical companies are globally known for their high standards in handling the microbiological hazards particularly (under the stewardship of the USFDA) in line with the ISO and HACCP compliance. Although no associated death is reported currently, 17 were hospitalized out of the 18 reported illnesses in 15 US states, the Centres for Disease Control and Prevention (CDC) reports.¹ Studies on the specific food that is potentially at risk as the precursor of the outbreak and the yet-to-ascertain responsible factors are being undertaken. Identifying the potential food source behind the outbreak was jointly investigated by the CDC and the United States Food and Drug Administration (USFDA) which included interviewing the patients on the kind of food they consumed before falling sick. The CDC found that an ice cream manufacturing house and its warehouse in Brooklyn, NY were supposedly the source or origin of the recent outbreak.² It was confirmed upon isolating the Listeria strain responsible for the outbreak from the “Soft Serve On The Go” vanilla chocolate cup ice cream that was invariably consumed by many patients. The NY State Department of Agriculture and Markets isolated Listeria strain from the ice cream cups at the manufacturing unit. An identical Listeria was also isolated from the environmental samples from vending stores of the ice cream.² Leafy greens and various branded packaged salads were also consumed before falling sick, the CDC reported.³

A similar Listeria outbreak attributed to the vanilla ice cream was reported in Florida in 2022, of which 28 persons were hospitalized and one died.⁴ As per the CDC, the US experienced about 30 such Listeria outbreaks in the last decade, with a total reported 524 listeriosis cases and 80 deaths (15.26% case fatality rate, CFR).⁵ The transmission sources included ice cream, leafy greens, mushroom, meat slice, dairy products like cheese, packaged salads, cooked chicken, hard-boiled egg, pork products, frozen vegetable, raw milk, packaged caramel apple, bean sprout, and soya products. The minimally processed food was potentially the source of Listeria and other food-borne bacterial (like Staphylococcus aureus and E. coli) and viral (like Norovirus) pathogens, it was opined.⁶ The recurrence of Listeria outbreak in the United States surprises as the aerobic bacterial food infections and intoxications cases are expected to be rare owing to the prevailing typical anaerobic food packaging practices there. Also, the food and pharmaceuticals companies, under the strict monitoring of the USFDA have set high benchmarks in line with ISO and HACCP compliance, especially in containing microbiological hazards.

Listeria outbreak is not limited to the United States only as several such outbreaks with unclear origin in South Africa, Australia, and Europe are also documented. This article has made a detailed analysis of the compiled literature about the various reported Listeria outbreaks, its relation with the most likely food source and draws conclusions and recommendations to counter such possible outbreaks in future so that better public healthcare and human wellness is ensured.

2 MATERIALS

The materials for the article were formulated by considering relevant reports on Listeria outbreaks recurrence in the United States through a comprehensive search of online literature and acquiring the data published by the WHO, CDC, PubMed, ScienceDirect and Web of Science. The information from the selected articles was reviewed, shortlisted and compiled to develop an article with a seamless flow of events and observations for easy understanding of the readers and draw conclusions. The cited literature was compiled manually and presented in the references section of the article as per the Journal guidelines.

3 COMPLICATIONS

Although rare, Listeria may lead to serious illness (like invasive listeriosis) or death, as evidenced from the high (15.26%) CFR noticed during the United States outbreaks in the last decade. Listeriosis is reportedly critically hazardous and medically complicated in the pregnant, the geriatric above 65 years, and in individuals with weak immunity. Being fatal, hospitalization is often necessary in the geriatric. The affected cases for Listeria infection in the United States were primarily the age-extremes (neonates and the geriatric) population, the pregnant and the immunocompromised.^7-9 Listeria causes premature birth, miscarriage or even neonatal death. Listeriosis symptoms like fever, muscle ache, tiredness, headache, stiff neck, confused state of mind, loss of balance or seizures are manifested usually 2 weeks after infection. Listeria monocytogenes is often associated with human infection among the many Listeria sp.

A study evaluated the food-borne infection risks in patients undergoing organ (kidney, liver, heart, and lungs) transplants in Switzerland between 2008 and 2018,¹⁰ and revealed that 88% of the confirmed microbial illnesses cases were due to Campylobacter and 10% were attributed to nontyphoidal Salmonella. Other isolated cases were attributed to Shigella, Yersinia and enteropathogenic E. coli. As all these pathogenic organisms are usually food-borne, the study concluded that post-transplant food-borne ailment was a critical cause of morbidity and mortality in such patients. A potential Listeria-induced endocarditis case was reported in a 74-year-old hemodialysis patient with bacteraemic L. monocytogenes, and¹¹ F-fluorodeoxyglucose-positron emission tomography-computed tomography confirmed endocarditis.¹² Although Listeria is not a usual causative agent of infective endocarditis, the high-risk immunosuppressed and the elderly are susceptible.¹³ Listeria epidemiology includes complex habitat like soil and water environment, and colonization in animals and humans. It causes invasive infections, especially in the debilitated and immunosuppressed (like transplant patients, pregnant women, individuals on long-term immunosuppressive drugs and neonates), and can stay silently in a human body with no symptoms. Listeria could lead to sepsis, meningitis and neurolisteriosis initially, that may lead to severe morbidity and mortality without timely interventions.^14-18

4 OUTBREAK CHARACTERIZATION

Listeria is a motile gram-positive non-sporulating bacillus. Believed to be an opportunistic pathogen, it is associated with infections like breast abscess, bacteraemia, meningoencephalitis and others.^{11, 19, 20} Due to its ability to thrive at low temperatures, frozen food is often associated with listeriosis. Listeria diagnosis is carried out through basic laboratory techniques, although it is complex and often lacks brevity.²¹ The motility, stress tolerance and adaptability of L. monocytogenes determine its virulence and aggressive invasion into the intestinal epithelial cells (that causes intestinal infection) and various other cell types.^22-24 Hypervirulent L. monocytogenes strains with an ability to infect respiratory system (the lungs) was reported recently in a coronavirus disease-19 (COVID-19) pandemic patient.²⁵ The whole genome sequencing (WGS) of L. monocytogenes isolated from cheese revealed that it was resistant to antimicrobials (AMR) like ciprofloxacin and meropenem, assumed to have acquired along the food chain.²⁶

5 ASSOCIATED FOOD VEHICLES

The United States has witnessed Listeria outbreaks earlier too, causing grim, life-threatening illnesses. Table 1 below is a summary of the numerous recently reported listeriosis outbreaks in the United States. Listeriosis was earlier linked primarily to the consumption of raw milk and its products.²⁷ Food-borne hazardous bacteria like Listeria, Salmonella, E. coli and Campylobacter are often found in raw unpasteurized milk.²⁸ The CDC recommended to avoid eating, serving or selling the recalled organic and traditional frozen fruits and vegetables and their products for households, restaurateurs and retailers. The consumption of pasteurized dairy product like milk, soft cheese, ice cream and yogurt is suggested, wherein the hazardous microbes are supposedly killed. This is more critical for those at greater risk, like the pregnant, individuals below 5 years and above 65 years, and the immunocompromised. Epidemiological and laboratory evidence of the 2016 outbreak specified unskilled handling of frozen vegetables as the likely source behind that health-emergency situation.²⁹ The 2018 outbreak was linked to Deli ham.³⁰ The 2020 outbreak was linked primarily to the hard-boiled egg.³¹ Listeriosis was linked to Deli meat and pork products earlier too.^{32, 33}

6 RISK FACTORS

The infection in the newborn and infants by a novel bacterium Cronobacter sakazakii was linked to their formula food consumption.^{34, 35} The organism was identified as an emerging opportunistic pathogen. An early onset of listeriosis in the newborn is evidenced by the presence of Listeria in the first 7 days postparturition. Majority of such cases result from teratogenic bacterial transfer to the newborn during vaginal delivery as the bacteria colonize the genitourinary tract.³⁶ Listeria transmission from mother to child was documented wherein 40 pregnant women and their children were evaluated for listeriosis by culturing their blood and cerebrospinal fluid (CSF). Clinical (fever and abdominal pain) and laboratory findings (elevated C-reactive protein in white blood cells and neutrophils) in the pregnant are potential listeriosis biomarkers.³⁷ Clinical signs in the newborn include respiratory distress and fever along with laboratory findings of raised C-reactive protein, and decreased glucose in the CSF. A French study on 189 neonates revealed that treating the listeriosis-suspected pregnant with antibiotics could prevent teratogenic transmission.³⁸ Pregnant lady exposed to a high-risk diet and exhibiting influenza-like illness may be identified and administered with antibiotics like amoxicillin.^{39, 40} Premature newborns, neonates born outside the healthcare facilities and low-birth-weight babies were increasingly at risk for an early onset of listeriosis.⁴¹

Bovine spongiform encephalopathy (BSE), also known as the mad cow disease is a central nervous system (CNS) disease caused by prion protein. It is a pathologically transformed normal human protein affecting the brain. Infectious prion proteins survive high temperature and warrants effective screening during food processing.⁴² Many other foodborne diseases in food processing/manufacturing establishments pose a biohazard risk. Threatening foodborne pathogens include E. coli, S. aureus, Campylobacter, Vibrio cholerae, V. parahemolyticus and Bacillus cereus among the numerous others. As Listeria resists low temperature, it could survive especially in the reasonably highly perishable processed, packaged and stored foods at low temperature through the supply chain for enhanced shelf-life. This enables the spread of Listeria, resulting in frequent food-borne outbreaks. Studies have identified a Listeria serovariants shift from serovar 4b in the 1990s to serovar 1/2a in the 2000s.⁴³ While assessing bulk-tank bovine milk, a Greek study found that 1/2a and 3a serovariants were prevalent. Several virulence genes like inlA, inlC, inlJ, iap, plcA, hlyA and actA occurred in these isolates.⁴⁴ That all isolated Listeria strains were multi-drug resistant is alarming. Deciphering Listeria ecosystem and evaluating its transmission dynamics is important. It is suggestible to use whole genome sequencing (WGS), metagenomics and bioinformatic approaches to assess epidemiological characteristics, antimicrobial resistance and virulence gene markers.⁴⁵

7 SUGGESTED RECOMMENDATIONS

Although Listeria outbreaks in South Africa, Australia and Europe were previously reported, the exact food source that was behind it has been unclear.^46-48 Recommendations to counter Listeria infections in the pregnant, the newborn, the geriatric above 65 years, the immunosuppressed individuals, and the patients with cancer, AIDS, organ transplant and nephrological clinical conditions were proposed.⁴⁹ They include hand-washing after handling fruits and vegetables and ready-to-eat food, washing the unprocessed raw fruits and vegetables under running water, maintaining processed food either below 5°C or above 60°C, storing the packed and cooked meat separately and timely discarding the unconsumed refrigerated and packaged foods.⁴⁹ Listeria could survive for up to 7 months (in bulk milk) to 7 years (in ice-cream), as per a recent report on various food sources. Clearly, demarcating the hygienic-handling (high-risk) and other environs (low-risk) zones in food processing facilities is suggested. Meticulous environmental screening to minimize the transfer of Listeria into the high-risk zone resulting in the contamination of packaged food is suggested.

Molecular techniques like real-time nucleic acid sequence-based amplification (NASBA), PCR, loop-mediated isothermal amplification (LAMP) and oligonucleotide-based microarrays, as also immunological methods like immunomagnetic capture and lateral flow immunoassay/immunochromatography are useful state-of-the-art screening and diagnostic tools. Optical, piezoelectric, electrochemical and cell-based biosensor approaches and spectrophotometric methods like near-infrared spectroscopy (NIR), Raman spectroscopy, matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) and hyperspectral imaging in manufacturing unit and surrounding environments are also recommended for effective screening.^50-52 Applying the thermal, non-thermal, chemical, phyto-extracted listeriostatic and listericidal agents to control Listeria is suggested. Using biopreservatives like essential oils, biological agents like Lactobacillus species, Enterococcus faecium, Staphylococcus xylosus, Debaryomyces hansenii and Penicillium chrysogenum, probiotics, and spices to inhibit the growth of pathogenic and to prevent microbial spoilage while retaining benign microbes in food packaging during manufacturing in line with the Hazard Analysis and Critical Control Point (HACCP) approach are suggested.^{53, 54}

8 SAFETY ISSUES

Retailers need to clean and sanitize food-display drawers, refrigerator shelves and freezers to avoid contamination. The USFDA recommends extra vigilance in cleaning and sanitizing food-contact surfaces that may be in direct or indirect contact with contaminated food products to reduce the risk of cross-contamination. Retailers and restaurateurs also need to sanitize and disinfect the food preparation, storage and serving areas to reduce the risk. At the same time, it is necessary that manufacturers strictly follow their in-house hygiene and safety measures as per their laid down SSOP (Sanitation Standard Operating Procedure) and further strengthen it for effective biohazard control. Rapid and timely updates on social media about such outbreaks, the most likely food source to remain vigilant about or even avoid, and self-protection measures from time to time are also suggested. Similar platforms could also be used to publish listeriosis-specific clinical features, symptoms and likely transmission chain for larger public reach.

9 CONCLUSION

The United States has witnessed Listeria outbreak with life-threatening consequence as discussed above. Due to typical anaerobic food packaging practices that prevail in the United States, recurring Listeria outbreaks in there certainly surprises. Under the USFDA monitoring, companies there maintain high standards, particularly in the food and pharmaceutical sectors, especially in containing microbiological hazards. Teratogenic listeriosis during vaginal delivery is a matter of concern. It is suggested to assess the epidemiological aspects, identify the gene markers of virulence and assay the antimicrobial resistance using whole genome sequencing (WGS), metagenomics and bioinformatics approaches, as well as numerous state-of-the-art techniques as suggested above. Hygiene and sanitation seem to play a critical role in nipping the ailment at bud. Further, religiously following the USFDA-set guidelines and maintaining high HACCP standards shall ensure that the process is fool-proof throughout the food supply chain. All in all, the Quality Management System (QMS) needs to abide by in true letter and spirit to effectively to specifically counter listeriosis and numerous other such foodborne microbiological hazards in general.

AUTHOR CONTRIBUTIONS

Ranjan K. Mohapatra: Conceptualization; project administration; writing—original draft. Snehasish Mishra: Writing—review & editing. Lawrence Sena Tuglo: Writing—original draft. Ashish K. Sarangi: Writing—original draft. Venkataramana Kandi: Writing—original draft. Amani Ahmed AL Ibrahim: Writing—review & editing. Hussain Abdulkhaliq Alsaif: Writing—original draft. Ali A. Rabaan: Supervision; Writing—review & editing. Md Kudrat-E Zahan: Writing—review & editing.

CONFLICT OF INTEREST STATEMENT

The authors declare no conflict of interest.

TRANSPARENCY STATEMENT

The lead author Venkataramana Kandi, Md. Kudrat-E Zahan affirms that this manuscript is an honest, accurate, and transparent account of the study being reported; that no important aspects of the study have been omitted; and that any discrepancies from the study as planned (and, if relevant, registered) have been explained.