Get to Know Pseudomonas Cocovenenans for Safe Soaking of Wood Ear Mushroom (Black Fungus) in Water


A severe and life-threatening food poisoning incident resulted from consumption of homemade cold wood ear mushroom was reported in recent months. Wood ear mushroom is a very common food ingredient in Chinese cuisine and is easily found in supermarkets and Chinese pharmacies. This fungus has a refreshing and crunchy texture; also, it is rich in dietary fibres so that it can enhance human intestinal peristalsis. It is a type of health-promoting food. However, why did it cause the food poisoning? In this case, bongkrekic acid was detected in the cold dish containing the wood ear mushroom and identified as the source of food poisoning incident. Bongkrekic acid is a major toxic metabolite produced by Pseudomonas cocovenenans subsp. farino fermentans (abbreviated to Pseudomonas cocovenenans).


Pseudomonas Cocovenenans and Bongkrekic Acid

Food items are susceptible to spoilage owing to many factors, such as humidity, hot weather, poor hygiene condition and improper storage. Food poisoning may occur as a consequence of eating spoiled food carelessly. In the aforementioned case of food poisoning, the wood ear mushroom was contaminated by pathogenic bacteria during the soaking process based on an official investigation. The bacterial proliferation was enhanced by leaving the fungi under improper conditions over an extended period. Pseudomonas cocovenenans in back fungus grows best at 37 and is able to produce the toxic bongkrekic acid at around 26. This acid is thermostable; also, its toxicity cannot be destroyed in boiling water or pressure cooker, nor can it be removed by washing or immersion. It is no specific antidote to treat bongkrekic acid poisoning currently, whose early symptoms include stomach discomfort, nausea, vomiting, abdominal distention and abdominal pain. In severe cases, the victim develops hepatomegaly, deranged liver function, hepatic encephalopathy or even death. Yet, the patients generally do not have fever.


What types of food are susceptible to contamination by Pseudomonas cocovenenans?

Bongkrekic acid has been proven to be present in spoiled fermented cereal and grain products, spoiled fresh white fungus (Tremella fuciformis) and other spoiled starchy products. Among them, the common food items are glutinous rice balls, potato noodles, glutinous rice cakes and sweet potato flour, etc. In the majority of food poisoning cases, ingestion of homemade fermented food or spoiled fresh white fungus was the cause. However, bongkrekic acid is unlikely produced when consumers soak the wood ear mushroom for only a short time under hygienic environment. Therefore, the fungi are safe for consumption if they have been properly soaked and cooked on the same day.


Precautions against Pseudomonas cocovenenans contamination to prevent it toxin production

Pseudomonas cocovenenans originates from the soil. The contamination is mainly due to lack of knowledge about proper food preparation and storage. After analysis, there are three factors attributed to the food poisoning incident, namely contamination by Pseudomonas cocovenenans, soaking over an extended period of time and poor hygiene condition. Although Pseudomonas cocovenenans are harmful, residents should not be too worried of it. We can avoid incident occur as we abide the following principles and strengthen our hygienic operation.

  • Buy good quality mushrooms and fungi. Do not buy those with an abnormal appearance, off-colour or off-odour;
  • Never use mouldy maize as raw material for making fermented dough products;
  • Soak enough amount of fungi for a meal in a short period of time and avoid soaking in liquid overnight;
  • Summer temperatures are favourable to the growth of Pseudomonas cocovenenans so it is advisable to soak fungi at refrigerated temperatures;
  • Discard any fungi that look abnormal or turn slimy after soaked;
  • Replace the water used for soaking the cereal and grain food products frequently to avoid development of unpleasant odour;
  • Keep the kitchen and countertops in good hygiene, clean and tidy. Wash the tableware promptly and do not leave food remnants behind. 

(Updated in March 2024)