Opportunistic Candida Infections in Critical COVID-19 Patients

Evaluation of candidemia cases in the intensive care unit of a tertiary training hospital during the period of COVID-19 pandemic

BACKGROUND

Many risk factors, including COVID-19 infection, lead to the development of invasive Candida infection in intensive care unit patients. The aim of this study was to evaluate the risk factors affecting mortality along with the clinical characteristics of candidemia patients.

METHODS

This retrospective study was conducted among patients hospitalized at the Anesthesiology and Reanimation Clinic between June 2020 and December 2021. The clinical and laboratory characteristics of 165 patients with candidemia were recorded. The difference between patients with and without COVID-19 infection was evaluated statistically. Multivariate analysis was performed to determine factors affecting mortality.

RESULTS

A total of 165 patients were included in the study, 52.1% of whom were male. The mean age of the patients was 66.5 (median 18-97) years. The percentage of patients with COVID-19 infection was 70.9%. The mean leukocyte count and aspartate transaminase, alanine transaminase, C-reactive protein, lactate dehydrogenase, ferritin, and D-dimer levels were significantly greater in COVID-19 patients than non COVID-19 patients (p < 0.05). The mortality rate in patients with candidemia was 80.2%. The presence of comorbidities, corticosteroid use, advanced age, and high ferritin and D-dimer levels negatively affected mortality, according to the multivariate analysis results. C. albicans was the most frequently isolated Candida species.

CONCLUSIONS

We detected higher mortality rates in patients with candidemia who were elderly, had comorbidities, received corticosteroid treatment and had elevated ferritin and D-dimer levels. When steroids are used, it is necessary to remember that this drug is a double-edged sword and to be careful of fungal infections.

Long COVID and gut candidiasis: What is the existing relationship?

Since the beginning of the coronavirus disease (COVID) 2019 pandemic, thousands of articles on the topic have been published, and although there is a growing trend of research on another associated condition, long coronavirus disease, important points still remain to be clarified in this respect. Robust evidence has suggested a relevant link between new clinical discoveries and molecular mechanisms that could be associated with the manifestations of different signs and symptoms involving cases of long COVID. However, one of the existing gaps that requires further investigation concerns a possible relationship between gut candidiasis and long COVID. While recent studies also suggest an interplay between the occurrence of these two conditions, it is not yet fully clear how this may happen, as well as the specifics regarding the possible pathophysiological mechanisms involved. In this connection and with the advent of a potential strengthening of the body of evidence supporting the hypothesis of a link between gut candidiasis and long COVID, a better understanding of the clinical presentation, pathophysiology and clinical management of such a relationship should be essential and useful for both, additional advances towards more targeted research and appropriate case management. Knowing more about the signs, symptoms, and complications associated with cases of long COVID is essential in order to more effectively mitigate the related burden and provide a higher quality of care and life for the affected population. In light of this and the need for better outcomes, here we review and discuss the content on different aspects of long COVID, including its pathophysiology and the existing evidence of a potential relationship between such a condition and gut candidiasis, as well as suggest propositions for future related research.

Candidemia Following Severe COVID-19 in Hospitalised and Critical Ill Patients: A Systematic Review and Meta-Analysis

RATIONALE

The epidemiology and clinical impact of COVID-19-associated candidemia (CAC) remained uncertain, leaving gaps in understanding its prevalence, risk factors and outcomes.

METHODS

A systematic review and meta-analysis were conducted by searching PubMed, Embase and Scopus for reports of CAC prevalence, risk factors and clinical outcomes up to June 18, 2024. The generalised linear mixed model was employed to determine the prevalence and 95% confidence intervals (CIs). The risk factors and clinical outcomes were compared between patients with and without CAC using the inverse variance method.

RESULTS

From 81 studies encompassing 29 countries and involving 351,268 patients, the global prevalence of CAC was 4.33% (95% Cl, 3.16%-5.90%) in intensive care unit (ICU) patients. In ICUs, the pooled prevalence of CAC in high-income countries was significantly higher than that of lower-middle-income countries (5.99% [95% Cl, 4.24%-8.40%] vs. 2.23% [95% Cl, 1.06%-4.61%], p = 0.02). Resistant Candida species, including C. auris, C. glabrata (Nakaseomyces glabratus) and C. krusei (Pichia kudriavzveii), constituted 2% of ICU cases. The mortality rate for CAC was 68.40% (95% Cl, 61.86%-74.28%) among ICU patients. Several risk factors were associated with CAC, including antibiotic use, central venous catheter placement, dialysis, mechanical ventilation, tocilizumab, extracorporeal membrane oxygenation and total parenteral nutrition. Notably, the pooled odds ratio of tocilizumab was 2.59 (95% CI, 1.44-4.65).

CONCLUSIONS

The prevalence of CAC is substantial in the ICU setting, particularly in high-income countries. Several risk factors associated with CAC were identified, including several that are modifiable, offering the opportunity to mitigate the risk of CAC.

Sero-prevalence of Toxoplasma gondii before and during the COVID-19 pandemic in Northwestern Iran

BACKGROUND

Toxoplasma gondii (T. gondii) is a ubiquitous protozoan parasite on our planet that causes toxoplasmosis. This study evaluated the seroprevalence and related risk factors for T. gondii infection in a population referred to healthcare centers in Meshkin-Shahr, Northwest Iran.

METHODS

A total of 400 blood samples were randomly collected from the general population and assessed using the anti-Toxoplasma antibodies, Immunoglobulin G and M (IgG and IgM) Enzyme-linked immunosorbent assay (ELISA) Kits in two steps before and during the coronavirus disease 2019 (COVID-19) pandemic, 2019-2020. The results were analyzed through logistic regression via SPSS 26 software.

RESULTS

Before the COVID-19 pandemic, anti-toxoplasma antibodies were detected in 39% of individuals (IgG: 38%, IgM: 0.5%, and IgG-IgM: 0.5%). Among the eleven risk factors evaluated, contact with soil and people awareness were significantly associated with T. gondii infection (p < 0.05). However, factors such as females, 20-39 age groups, junior high schools, housewives, rural areas, raw meat or vegetable consumption, vegetable or fruits washed by water, not detergent, and cat owners did not show a significant relationship with seropositivity (p > 0.05). After the outbreak of the COVID-19 pandemic, the overall seroprevalence for anti-T. gondii antibody increased to 49.7% (IgG: 47.7%, IgM: 0.5%, and IgG and IgM: 1.5%). Among these patients, 26% were positive for COVID-19. Additionally, before the COVID-19 pandemic, 40 samples were negative for anti-T. gondii antibodies but later became positive. The crude and adjusted models suggested that toxoplasmosis may be a possible risk factor for increased susceptibility to COVID-19, with an odds ratio (OR) of 1.28 (95% confidence interval (CI), 0.82-1.99; P < 0.05). Conversely, a non-significant protective effect against latent toxoplasmosis was observed in COVID-19-positive individuals (OR = 0.99; 95% CI, 0.51-1.92; P > 0.05), and COVID-19 positivity did not increase the levels of anti-T. gondii IgG antibodies.

CONCLUSIONS

The general population in this region had a moderate seroprevalence of T. gondii. The increased number of COVID-19-positive patients with latent toxoplasmosis highlights the need to pay attention to the early diagnosis and proper treatment of toxoplasmosis in these patients and implement preventive programs in these areas for future possible viral infections.

Hospitalized COVID-19 Patients with Urinary Tract Infection in Iran: Candida Species Distribution and Antifungal Susceptibility Patterns

Candida species, typically part of the human skin and mucous membrane flora, can cause opportunistic fungal infections, notably urinary tract infections (UTIs), which are on the rise among hospitalized COVID-19 patients. The lack of understanding of UTIs in this population, coupled with the emergence of multidrug-resistant strains, poses significant challenges for effective treatment and further investigations. In this study, urine samples were collected from 70 COVID-19 patients with UTIs in sterile containers for microbiology examination. After microscopic observation, the isolates were identified both by phenotypic and molecular techniques such as multiplex PCR. Antifungal susceptibility testing (AFST) against fluconazole (Flu), itraconazole (Itr), and amphotericin B (AMB) was performed according to CLSI M27/S4 standard methods, with the frequency of isolates including Candida albicans (n = 20, 51.3%), Candida tropicalis (n = 15, 38.4%), Nakaseomyces glabrata (previously Candida glabrata) (n = 2, 5.1%), Pichia kudriavzevii (previously Candida krusei), and Candida parapsilosis (n = 1, 2.5%). All isolates of C. albicans, C. tropicalis, C. glabrata, and C. parapsilosis were sensitive to amphotericin B, while C. kruzei was resistant to AMB. Around 70% of C. albicans isolates were sensitive to Flu; 20% of C. tropicalis were resistant to itraconazole, while 33% were resistant to fluconazole. C. albicans and C. tropicalis were the main causes of candiduria in infected cases and both Flu and AMB showed good results in AFST in these species. Performing drug susceptibility testing for clinical isolates of Candida spp. provided guidance for appropriate management and control, and timely antifungal treatment.

A prediction model for secondary invasive fungal infection among severe SARS-CoV-2 positive patients in ICU

Background

The global COVID-19 pandemic has resulted in over seven million deaths, and IFI can further complicate the clinical course of COVID-19. Coinfection of COVID-19 and IFI (secondary IFI) pose significant threats not only to healthcare systems but also to patient lives. After the control measures for COVID-19 were lifted in China, we observed a substantial number of ICU patients developing COVID-19-associated IFI. This creates an urgent need for predictive assessment of COVID-19 patients in the ICU environment for early detection of suspected fungal infection cases.

Methods

This study is a single-center, retrospective research endeavor. We conducted a case-control study on severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) positive patients. The cases consisted of patients who developed any secondary IFI during their ICU stay at Jilin University China-Japan Union Hospital in Changchun, Jilin Province, China, from December 1st, 2022, to August 31st, 2023. The control group consisted of SARS-CoV-2 positive patients without secondary IFI. Descriptive and comparative analyses were performed, and a logistic regression prediction model for secondary IFI in COVID-19 patients was established. Additionally, we observed an increased incidence of COVID-19-associated pulmonary aspergillosis (CAPA) during this pandemic. Therefore, we conducted a univariate subgroup analysis on top of IFI, using non-CAPA patients as the control subgroup.

Results

From multivariate analysis, the prediction model identified 6 factors that are significantly associated with IFI, including the use of broad-spectrum antibiotics for more than 2 weeks (aOR=4.14, 95% CI 2.03-8.67), fever (aOR=2.3, 95%CI 1.16-4.55), elevated log IL-6 levels (aOR=1.22, 95% CI 1.04-1.43) and prone position ventilation (aOR=2.38, 95%CI 1.15-4.97) as independent risk factors for COVID-19 secondary IFI. High BMI (BMI ≥ 28 kg/m2) (aOR=0.85, 95% CI 0.75-0.94) and the use of COVID-19 immunoglobulin (aOR=0.45, 95% CI 0.2-0.97) were identified as independent protective factors against COVID-19 secondary IFI. The Receiver Operating Curve (ROC) area under the curve (AUC) of this model was 0.81, indicating good classification.

Conclusion

We recommend paying special attention for the occurrence of secondary IFI in COVID-19 patients with low BMI (BMI < 28 kg/m2), elevated log IL-6 levels and fever. Additionally, during the treatment of COVID-19 patients, we emphasize the importance of minimizing the duration of broad-spectrum antibiotic use and highlight the potential of immunoglobulin application in reducing the incidence of IFI.

Epidemiological Assessment and Risk Factors for Mortality of Bloodstream Infections by Candida sp. and the Impact of the COVID-19 Pandemic Era

Candidemia is one of the healthcare-associated infections that has high mortality. The risk factors that predispose a patient to develop this infection are mostly found in patients of greater severity and COVID-19 contributes to the risk of death. The aim of this study is to evaluate epidemiological characteristics and risk factors for mortality in patients with candidemia before and during the COVID-19 pandemic era. This is a retrospective study conducted at Instituto Central from 2016 to 2020 of patients with candidemia that were evaluated for demographic data, medical history, risk factors, microbiological data, therapeutic measures, complementary exams, device management, and outcome defined by 30-day mortality. A total of 170 episodes were included (58.2% males; mean age of 56 years). The overall incidence density of candidemia per 1000 admissions and per 1000 patient-days was 1.17 and 0.17, respectively, with an increase of 38% in the year 2020. The use of a central venous catheter was the most prevalent (93.5%) condition, followed by the previous use of antibiotics (91.1%). Corticosteroid use ranked seventh (56.4%). C. albicans was responsible for 71 (41.7%) of the isolates, followed by C. tropicalis and C. glabrata, with 34 (20%) isolates each. Echinocandin was prescribed in 60.1% of cases and fluconazole in 37%. Echocardiography resulted in six (5.08%) cases of endocarditis and fundoscopy resulting in two (2.4%) endophthalmitis. The 30-day mortality was 93/170 (54.7%). The risk factors associated with mortality were age (OR 1.03, CI 95% 1.01-1.06), heart disease (OR 7.51, CI 95% 1.48-37.9), hemodialysis (OR 3.68, CI 95% 1.28-10.57), and use of corticosteroids (OR 2.83, CI 95% 1.01-7.92). The COVID-19 pandemic had an impact on the increase incidence of candidemia. The persistently high mortality highlights the need for better management strategies, control of risk factors, and guarantee of adequate treatment.

Total Infectome Characterization of Respiratory Infections during the 2022-23 COVID-19 Outbreak in China Revealed Extensive Coinfections with Links to SARS-CoV-2 Status, Age, and Disease Severity

Between 7 December 2022 and 28 February 2023, China experienced a new wave of COVID-19 that swept across the entire country and resulted in an increasing amount of respiratory infections and hospitalizations. The purpose of this study is to reveal the intensity and composition of coinfecting microbial agents. In total, 196 inpatients were recruited from The Third People's Hospital of Shenzhen, and 169 respiratory and 73 blood samples were collected for metagenomic next-generation sequencing. The total "Infectome" was characterized and compared across different groups defined by the SARS-CoV-2 detection status, age groups, and severity of disease. Our results revealed a total of 22 species of pathogenic microbes (4 viruses, 13 bacteria, and 5 fungi), and more were discovered in the respiratory tract than in blood. The diversity of the total infectome was highly distinguished between respiratory and blood samples, and it was generally higher in patients that were SARS-CoV-2-positive, older in age, and with more severe disease. At the individual pathogen level, HSV-1 seemed to be the major contributor to these differences observed in the overall comparisons. Collectively, this study reveals the highly complex respiratory infectome and high-intensity coinfection in patients admitted to the hospital during the period of the 2023 COVID-19 pandemic in China.

Metagenetic analysis of the bacterial diversity of Kazakh koumiss and assessment of its anti-Candida albicans activity

Koumiss, a five-thousand-year-old fermented mare's milk beverage, is widely recognized for its beneficial nutrient and medicinal properties. The microbiota of Chinese and Mongolian koumiss have been largely characterized in recent years, but little is known concerning Kazakh koumiss despite this drink historically originates from the modern Kazakhstan territory. In addition, while koumiss is regarded as a drink with therapeutic potential, there are also no data on koumiss anti-Candida activity. In this context, the aims of the present study were to investigate the bacterial diversity and anti-Candida albicans activity of homemade Kazakh koumiss samples as well as fermented whey and cow's milk, derived from koumiss and propagated for several months. Koumiss bacterial communities were largely dominated by lactic acid bacteria including Lactobacillus sensu lato spp. (69% of total reads), Streptococcus (8.0%) and Lactococcus (6.1%), while other subdominant genera included Acetobacter (2.6%), Enterobacter (2.4%), and Klebsiella (1.5%). Several but not all koumiss samples as well as fermented whey and cow's milk showed antagonistic activities towards C. albicans. Linear discriminant effect size (LEfSe) analysis showed that their bacterial communities were characterized by a significantly higher abundance of amplicon sequence variants (ASV) belonging to the genus Acetobacter. In conclusion, this study allowed to identify the key microorganisms of Kazakh koumiss and provided new information on the possible underestimated contribution of acetic acid bacteria to its probiotic properties.

Fungal microbiota sustains lasting immune activation of neutrophils and their progenitors in severe COVID-19

Gastrointestinal fungal dysbiosis is a hallmark of several diseases marked by systemic immune activation. Whether persistent pathobiont colonization during immune alterations and impaired gut barrier function has a durable impact on host immunity is unknown. We found that elevated levels of Candida albicans immunoglobulin G (IgG) antibodies marked patients with severe COVID-19 (sCOVID-19) who had intestinal Candida overgrowth, mycobiota dysbiosis and systemic neutrophilia. Analysis of hematopoietic stem cell progenitors in sCOVID-19 revealed transcriptional changes in antifungal immunity pathways and reprogramming of granulocyte myeloid progenitors (GMPs) for up to a year. Mice colonized with C. albicans patient isolates experienced increased lung neutrophilia and pulmonary NETosis during severe acute respiratory syndrome coronavirus-2 infection, which were partially resolved with antifungal treatment or by interleukin-6 receptor blockade. sCOVID-19 patients treated with tocilizumab experienced sustained reductions in C. albicans IgG antibodies titers and GMP transcriptional changes. These findings suggest that gut fungal pathobionts may contribute to immune activation during inflammatory diseases, offering potential mycobiota-immune therapeutic strategies for sCOVID-19 with prolonged symptoms.

Candidiasis: From cutaneous to systemic, new perspectives of potential targets and therapeutic strategies

Candidiasis is an infection caused by fungi from a Candida species, most commonly Candida albicans. C. albicans is an opportunistic fungal pathogen typically residing on human skin and mucous membranes of the mouth, intestines or vagina. It can cause a wide variety of mucocutaneous barrier and systemic infections; and becomes a severe health problem in HIV/AIDS patients and in individuals who are immunocompromised following chemotherapy, treatment with immunosuppressive agents or after antibiotic-induced dysbiosis. However, the immune mechanism of host resistance to C. albicans infection is not fully understood, there are a limited number of therapeutic antifungal drugs for candidiasis, and these have disadvantages that limit their clinical application. Therefore, it is urgent to uncover the immune mechanisms of the host protecting against candidiasis and to develop new antifungal strategies. This review synthesizes current knowledge of host immune defense mechanisms from cutaneous candidiasis to invasive C. albicans infection and documents promising insights for treating candidiasis through inhibitors of potential antifungal target proteins.

Fungal Infections Identified with Multiplex PCR in Severe COVID-19 Patients during Six Pandemic Waves

Background and Objectives: With an increasing number of severe COVID-19 cases presenting with secondary fungal infections, this study aimed to determine the prevalence of fungal co-infections in severe COVID-19 patients across the six waves, identify the most common fungal pathogens associated with severe COVID-19, and explore any potential links between patient characteristics, therapeutic strategies, and the prevalence and type of fungal infection. Materials and Methods: A retrospective analysis was conducted on severe COVID-19 patients admitted to the Infectious Diseases and Pulmonology Hospital, "Victor Babes", Romania, between March 2020 and August 2022. Samples were collected from respiratory specimens, blood, and urine, after which a standard nucleic acid extraction protocol was employed. Patients were divided into groups with and without fungal infections, identified using multiplex PCR. The groups were compared based on demographic data, comorbidities, pandemic wave number, and clinical outcomes. Results: Out of 288 patients, 96 (33.3%) had fungal infections, with Candida spp. being the most common. Patients with fungal infections had higher rates of obesity (35.4% vs. 21.4%, p = 0.010) and a higher Charlson comorbidity index (CCI > 2) (37.5% vs 25.0%, p = 0.027). Ventilator use was significantly higher in the fungal infection group (45.8% vs. 18.8%; p < 0.001), as was ICU admission (39.6% vs. 26.6%; p = 0.024) and mortality (32.3% vs 12.0%; p < 0.001). The distribution of different fungal species varied across the pandemic waves, with no statistical significance (p = 0.209). The mortality risk notably increased with the degree of drug resistance (OR for three or more drug resistances = 6.71, p < 0.001). The second, fourth, and fifth pandemic waves were significantly associated with higher mortality risk (OR = 3.72, 3.61, and 4.08, respectively, all p < 0.001). Aspergillus spp. and Mucor spp. infections were significantly associated with increased mortality risk (OR = 4.61 and 6.08, respectively, both p < 0.001). Conclusions: Our study indicates a significant presence of fungal co-infections among severe COVID-19 patients that is associated with increased morbidity and mortality, particularly in patients with drug-resistant infections. These findings underline the necessity for comprehensive diagnostic approaches and tailored treatment strategies in managing COVID-19 patients, especially during specific pandemic waves and in patients with particular fungal infections. Further research is required to understand the implications of these co-infections and their management.

The Clinical Characteristics of Bloodstream Infections Due to Candida spp. in Patients Hospitalized in Intensive Care Units during the SARS-CoV-2 Pandemic: The Results of a Multicenter Study

Candidemia is a serious health threat. Whether this infection has a greater incidence and a higher mortality rate in patients with COVID-19 is still debated. In this multicenter, retrospective, observational study, we aimed to identify the clinical characteristics associated with the 30-day mortality in critically ill patients with candidemia and to define the differences in candidemic patients with and without COVID-19. Over a three-year period (2019-2021), we identified 53 critically ill patients with candidemia, 18 of whom (34%) had COVID-19 and were hospitalized in four ICUs. The most frequent comorbidities were cardiovascular (42%), neurological (17%), chronic pulmonary diseases, chronic kidney failure, and solid tumors (13% each). A significantly higher proportion of COVID-19 patients had pneumonia, ARDS, septic shock, and were undergoing an ECMO procedure. On the contrary, non-COVID-19 patients had undergone previous surgeries and had used TPN more frequently. The mortality rate in the overall population was 43%: 39% and 46% in the COVID-19 and non-COVID-19 patients, respectively. The independent risk factors associated with a higher mortality were CVVH (HR 29.08 [CI 95% 3.37-250]) and a Charlson's score of > 3 (HR 9.346 [CI 95% 1.054-82.861]). In conclusion, we demonstrated that candidemia still has a high mortality rate in patients admitted to ICUs, irrespective of infection due to SARS-CoV-2.

Fungal Infections in the ICU during the COVID-19 Pandemic in Mexico

Background: Invasive Fungal Infections (IFI) are emergent complications of COVID-19. In this study, we aim to describe the prevalence, related factors, and outcomes of IFI in critical COVID-19 patients. Methods: We conducted a nested case-control study of all COVID-19 patients in the intensive care unit (ICU) who developed any IFI and matched age and sex controls for comparison (1:1) to evaluate IFI-related factors. Descriptive and comparative analyses were made, and the risk factors for IFI were compared versus controls. Results: We found an overall IFI prevalence of 9.3% in COVID-19 patients in the ICU, 5.6% in COVID-19-associated pulmonary aspergillosis (CAPA), and 2.5% in invasive candidiasis (IC). IFI patients had higher SOFA scores, increased frequency of vasopressor use, myocardial injury, and more empirical antibiotic use. CAPA was classified as possible in 68% and 32% as probable by ECMM/ISHAM consensus criteria, and 57.5% of mortality was found. Candidemia was more frequent for C. parapsilosis Fluconazole resistant outbreak early in the pandemic, with a mortality of 28%. Factors related to IFI in multivariable analysis were SOFA score > 2 (aOR 5.1, 95% CI 1.5-16.8, p = 0.007) and empiric antibiotics for COVID-19 (aOR 30, 95% CI 10.2-87.6, p = <0.01). Conclusions: We found a 9.3% prevalence of IFIs in critically ill patients with COVID-19 in a single center in Mexico; factors related to IFI were associated with higher SOFA scores and empiric antibiotic use for COVID-19. CAPA is the most frequent type of IFI. We did not find a mortality difference.

Lower Concentrations of Amphotericin B Combined with Ent-Hardwickiic Acid Are Effective against Candida Strains

Life-threatening Candida infections have increased with the COVID-19 pandemic, and the already limited arsenal of antifungal drugs has become even more restricted due to its side effects associated with complications after SARS-CoV-2 infection. Drug combination strategies have the potential to reduce the risk of side effects without loss of therapeutic efficacy. The aim of this study was to evaluate the combination of ent-hardwickiic acid with low concentrations of amphotericin B against Candida strains. The minimum inhibitory concentration (MIC) values were determined for amphotericin B and ent-hardwickiic acid as isolated compounds and for 77 combinations of amphotericin B and ent-hardwickiic acid concentrations that were assessed by using the checkerboard microdilution method. Time–kill assays were performed in order to assess the fungistatic or fungicidal nature of the different combinations. The strategy of combining both compounds markedly reduced the MIC values from 16 µg/mL to 1 µg/mL of amphotericin B and from 12.5 µg/mL to 6.25 µg/mL of ent-hardwickiic acid, from isolated to combined, against C. albicans resistant to azoles. The combination of 1 µg/mL of amphotericin B with 6.25 µg/mL of ent-hardwickiic acid killed all the cells of the same strain within four hours of incubation.