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Moniliella acetoabutens is a member of the Moniliella genus. It closely resembles fungi found in the Monilia genus; however due to morphological and environmental discrepancies it is classified differently. M. acetoabutens is characterized by thick cell walls and dark coloured cultures, while fungi found in the Monilia genus lack such features. M. acetoabutens is a unique fungus in that it can survive extremely acidic environments and can use acetic acid as a nutrient source, a characteristic rare among fungi. As a result, the specific epithet for acetoabutens abutor meaning "to use a thing which is consumed in the use" was chosen for M. acetoabutens. The ability of M. acetoabutens to survive in acidic conditions allows it to endure common preservative techniques and spoil a unique array of food products.

History and taxonomy
M. acetoabutens was discovered in the United Kingdom (UK) in 1958 by J.C. Dakin from spoiled sweet fruit sauce. It was fully characterized and officially named in 1966 by Stolk A.C & Dakin J.C.  It is believed the official discovery of M. acetoabutens was in 1949 by researchers Fabian and Faville who isolated the fungus from mold attributed to pickle spoilage. It was initially given the misapplied name Geotrichum candidum. The error is thought to have occurred due to a lack of known fungi existing within the Moniliella genus at the time of discovery. After 15 years of study Stolk and Dakin have reported only 10 incidences of M. acetoabutens induced food spoilage. However, the researchers claim that the incidence rate is increasing. Of the reported food spoilage cases 4 have occurred in brown fruit sauce, 3 in mint sauce, 1 in sweet pickles, 1 in gherkins and 1 in malt vinegar. No pattern in geographical distribution was determined but is was ascertained that bulk storage vessels are the most likely sources of contamination. It was later determined that within the Moniliella genus, M. acetoabutens is closely related to Moniliella suaveolans. Yet M. acetoabutens is unique in that it can survive in much more acidic environments as opposed to M. suaveolans which prefers substrates rich in oils like butter and margarine.

Isolated strains

 * Strain CBS 169.66 isolated from sweet fruit sauce containing 15% sucrose and 3% acetic acid in 1958 by J.C Dakin in England.
 * Strain CBS 170.66 isolated from fruit sauce in 1965 by J.D Adock and J. Colman in Norwich, England.
 * Strain CBS 171.66 isolated from sweet pickle made with corn syrup in 1960 by John L. Etchells in North Carolina, United States.

Growth and morphology
In order for optimal sporulation to occur in M. acetoabutens the fungus must be grown on malt extract agar (MEA) or a similar derivative at 20-22°C, where it demonstrates a radial growth pattern. The derived fungal colony is often described as having a diameter of 3-4cm within 7 days. If the fungus is grown on Czapek yeast autolysate (CYA), growth is more restrictive resulting in a smaller diameter. A colony of M. acetoabutens is often described as having white-brownish color at the edges with a more greenish-brown pigment towards the center of the colony. The colony is often covered in lanate (fine hairlike filaments), which can vary per strain. A distinguishing factor for fungal colonies of M. acetoabutens is that it demonstrates tensile strength. As it continues to grow the hyphae have a high tendency to fragment;  the resulting fragment is typically 150μm in length with thick walls having a width of 3.5-4.5μm. In addition, thick walled intercalary chlamydoconidia have been observed, allowing for moderate heat resistance. The microcolonies that arise from the fragmented hyphae also produce hyphae with branches at regular angles of 45°. During sporulation the conidia of M. acetoabutens demonstrate the following traits: those that arise apically are broadly ellipsoidal (4.5-9 X 3.5-6μm), while conidia that are derived laterally are often cylindrical with slightly rounded ends achieving a length of 10μm. More specifically colonies of M. acetoabutens have been identified as having chlamydospores, arthrospores, and blastospores. Chlamydospores are often seen when M. acetoabutens is growing in unfavorable conditions, such as extremely high temperatures. Blastospores have been seen to develop as "blow-out" ends that protrude away from the central hyphae it is on. Arthrospores have been characterized as abundant "chain-looking" thin walled growths that protrude from the developing hyphal tip. These characteristics may differ slightly depending on the strain type grown.

Specific observations by Stolk & Dakin on strain CBS 169.66

M. acetoabutens was grown on a malt extract for 1 month at 20°C. After 3 days of growth, thick loose pellicle and small dry areas of mycelium and pseudomycelium were observed. Free cells were identified and classified as round/oval blastospores (3.5-6.6μm X 4.5-7.8μm). After a month of growth the pellicle occupied half of the liquid contents, myceilum and pseudomycelium were still apparent and arthrospores were present at an average size of 5.5 X 7.5μm.

Physiology
M. acetoabutens is defined as a true yeast and can use glucose, sucrose and maltose as sources of energy to carry out fermentation. This fungus cannot use D-galactose, lactose and raffinose as sources of energy. Optimal growth for M. acetoabutens is between 20-22°C, with a maximum growth temperature of 37°C. No growth is found at 5°C.

Habitat and ecology
Due to the acidophilic nature of M. acetoabutens it can survive in areas containing moderate levels of acetic acid. As a consequence, M. acetoabutens has the capacity to contaminate pickles, vinegar, mayonnaise, fruit sauces, mint sauces, and salad dressings. Though the fungus is not said to have a defined geological location, it generally prefers a more Northern climate (cooler temperatures that do not exceed 30°C typically). M. acetoabutens was later found to be resistant to sorbic acid, which is commonly used in food preservation. M. acetoabutens does not produce mycotoxins and there are no recorded incidences of infection.

Detection
Stolk and Dakin developed a technique in order to determine if a food product has been contaminated by M. acetoabutens. A portion of the suspected food is plated on malt acetic agar or malt extract agar containing 2-4% acetic acid. If the food product is positive for M. acetoabutens rapidly growing white colonies will appear. Vinegar containing food products contaminated with M. acetoabutens will also give off a fruity odour, which can act as an additional indicator.

Preservation
Typically concentrations of acetic acid of 3-6% are sufficient to prevent spoilage by most fungi, however M. acetoabutens can tolerate up to 5%. Due to the resistance exhibited on acetic acid other compounds were tested by Stolk and Dakin to find a more suitable preservative. It was found that methyl para-hydroxybenzoate was unable to inhibit growth at any concentration while 100ppm of sulfur dioxide, 300ppm of propyl para-hydroxybenzoate,  and 400ppm of scorbic acid were sufficient. Nevertheless The Preservatives in Food Regulation (1962) only permits the use of 100ppm of sulfur dioxide and 250ppm of methyl/propyl para-hydroxybenzoate, much less than what is needed for successful preservation. Therefore it was concluded that high amounts of scorbic acid would be the most likely candidate for preservation against M. acetoabutens (where legally permitted). Other techniques that can be used to counter M. acetoabutens are thermal pasteurization and filtering. However thermal pasteurization will decrease the taste of the product and filtering is not possible for foods that contain solid material (i.e. salad dressing).

Australian mayonnaise spoilage (1971)
M. acetoabutens caused fermentative spoilage of a large production of mayonnaise in Australia in 1971. The contamination was found to be caused by wooden vinegar tanks being shipped in close proximity to the mayonnaise. The fungus was found growing on the wood of the vinegar tanks which contained 10% acetic acid. The growth was thought to be enhanced by the fact that the acetic acid began to evaporate as the temperature changed, resulting in cross contamination.