Microbial Contamination Of Vended Fruit

Viral pathogens associated with fruit contamination include:

1.    Mucor, Aspergillus, Penicillium, Rhizopus species

These organisms are soil inhabitants and could contaminate the fruits during post-harvesting. Moisture content appeared to be one of the major factors that support fungal growth in dates (Hill and Waller, 1999) as both the semi dry and soft types had all the six species identified from them. Storage facilities such as sacks, polythene bags and natural fibre, which are air-tight, should used for storage of all the varieties of fruits and vegetables that might have encouraged the growth of such species. 

2.    Norwalk-like viruses

Outbreaks of NLV are common in children and adults of all ages in hospitals, nursing homes, hotels and institutions where the virus is usually transmitted person to person. The virus can be spread in aerosols and the infectious dose is considered to be very low (10-100 virus particles). Food-borne outbreaks have been associated with infected food handlers and with shellfish or vegetables contaminated by human sewage (Hedberg and Osterholm, 1993; MMWR, 2001).

2.8. SOURCES OF MICROBIAL CONTAMINATION OF FRUITS.

The sources of fruit contamination can occur through the following methods or ways:

• Use of Organic fertilizers
• State of irrigation water
• Soil
• Harvesting procedure 

Use of Organic fertilizers:

The use of organic fertilizers, such as animal manures and slurries (Beuchat, 1996; Natvig et al., 2002), abattoir wastes (Avery et al., 2005) and sewage sludge (Al-Ghazali and Al-Azawi, 1990) introduce pathogens directly to the field, and run-off can contaminate irrigation water. There are comprehensive guidelines available to growers that advise on sufficient treatment of wastes and correct timing of application, with the aim of limiting contamination of crops. Intervals of application of these organic fertilizers should beset and regulated to ensure the microbial quality of produce at harvest. Similar recommendations are set out in the United States by the United States Environment Protection Agency Part (Anon 1996).

State of irrigation water

Faecal material, soil and other inputs such as sewage overflow introduce entero-pathogens directly to water courses from which irrigation water may be extracted. Irrigation water is mostly obtained from surface waters, which receive treated sewage effluent (Tyrell et al., 2006). The potential for contamination via irrigation water is increased in the developing world, as untreated wastewater is used for irrigation of around 10% of crops (Anon, 2003). Wastewater irrigated crops show an increased incidence of enteropathogens (Steele and Odemeru, 2004). Wachtel et al., (2002) described E. coli contamination of the roots of cabbage irrigated with sewage-contaminated stream water, although the edible part of the plant was unaffected. Islam et al., (2004) demonstrated that a single application of Salmonella typhimurium inoculated irrigation water resulted in contamination of carrot and radish at harvest, with Salmonella surviving for 203 days in soil post-application. The interval between irrigation and harvest will affect the likelihood of pathogenic bacteria surviving to reach the consumer.

Soil

Pathogens may be naturally present in soil, for example Listeria spp. (Nicholson et al., 2005), or may become incorporated in the soil matrix from organic wastes added as fertilizer. Pathogens within soil may contaminate crops directly when heavy rain or water gun irrigation causes leaf splash. The ability of the pathogen to survive in the environment will impact on the likelihood of crop contamination and pathogen viability at harvest and through to consumption. Initially, the pathogen must survive in the propagation environment until crops are planted out, or in organic wastes applied to the land. Kudva et al., (1998) demonstrated that aeration of ovine manure decreased survival of E. coli O157:H7 within 120 days. The application method used for organic wastes may increase survival time: clumping of material applied above ground, and injection application of liquid manures can protect bacteria from desiccation and high temperatures (Hutchison et al., 2004).
Cross-protection mechanisms may extend bacterial survival in the environment, by reducing the impact of abiotic factors. Leyer and Johnson, (1993) reported that after acid adaptation, Salmonella typhimurium displayed increased tolerance of heat and osmotic stress, while Hartke et al., (1995) demonstrated that pre-irradiation of Lactococcus lactis increased resistance to lethal challenges of acid. Seasonal variation in shedding of pathogens can result in higher than expected microbial loads in faecal material.

Harvesting procedure 

Post harvest treatment of fruits and vegetables includes handling, storage, transportation and cleaning. During these practices conditions may arise which lead to cross contamination of the produce from other agricultural materials or from the workers. Environmental conditions and transportation time will also influence the hygienic quality of the produce prior to processing or consumption. Poor handling can damage fresh produce, rendering the product susceptible to the growth or survival of spoilage and pathogenic microorganisms. This damage can also occur during packaging and transport. The presence of cut and damaged surfaces provides an opportunity for contamination and growth of microorganisms and ingress into plant tissues (Francis and O’Beirne, 1999). The first washing of vegetables at harvest removes much of the adhering soil and dirt.

However, it should be recognised that washing may also be a source of microbial contamination. An example is the contamination of tomatoes with Salmonella javania and of parsley with Shigella sonnei, which caused large outbreaks in the US (CDC, 2005). Mangoes exported to the US from Brazil were found to be infecting consumers with Salmonella (Sivapalasingam et al., 2000). However, there was no evidence that consumers in Europe were being infected. The infection was traced to contaminated hot water used to kill fruit-flies in the mangoes. Washing helps to reduce the microbial load of raw fruits and vegetables. 

Contamination can also occur through the 
•    Use of unhygienic water for washing
•    Prolong preservation, 
•    Processing in an unhygienic surrounding often with swarming houseflies, and 
•    Exposure to air-borne dust.

2.9. OUTBREAKS OF FOOD-BORNE DISEASES ASSOCIATED WITH THE CONSUMPTION OF CONTAMINATED FRUITS.

Vegetables and fruits have been associated with outbreaks of food-borne disease in many countries. Organisms involved include bacteria, viruses and parasites (De Roever, 1998). Lindqvist et al., (2000) reported that the outbreaks of foodborne infectious intestinal diseases were associated with the consumption of contaminated salad items, fruits and vegetables. Micro-organisms like Escherichia coli, Shigella, Salmonella, Vibrio cholerae and amoebas were isolated. The lack of robust traceability and weaknesses in reporting systems for outbreaks limits any comprehensive evaluation of the role of fruits and vegetables as a source of food-borne infections. 

    The minimum processing required for fresh and fresh-cut produce, which omits any effective microbial elimination step, results in food products that naturally would carry microorganisms, some of which may be potentially hazardous to human health. When investigating possible control methods, a vital step is to examine the nature of the human pathogenic microorganisms present in produce throughout the production process. However, incidence studies are time-consuming and expensive. For this reason, sample sizes are often too small to be of statistical relevance, especially if the probability of detection is low. Most researchers do not collect sufficient information regarding the source of the sample other than perhaps the country of origin or sample location (for example, retail outlets, and farmers’ markets). There has been little consistency in sample collection, treatment, laboratory test methods, or data analysis. Controls are often missing and techniques for isolating pathogens from produce items are often not optimized. In many cases, identification of the pathogen has not been verified. Most published articles stress the detection of pathogens in incidence surveys; negative data may not be reported or their significance is minimized (Wells and Butterfield, 1997).

 However, these negative data are important in evaluating the risks associated with consumption of fresh fruits and vegetables and should be considered in risk assessments. Because of the extremely large number of variables that might influence contamination of raw fruits or vegetables, it is difficult to design well-controlled experiments that would address risk factors for contamination. While incidence studies can provide a snapshot assessment of contamination at a particular location on a particular produce item at a particular time of year, they rarely provide information on the source of contamination. For these reasons, caution must be used when interpreting data from these types of studies, and overly broad conclusions should be avoided. Nevertheless, numerous pathogenic microorganisms have been isolated from a wide variety of fresh fruits and vegetables, sometimes at relatively high frequencies (Wells and Butterfield, 1999).
The number of food-borne illness outbreaks linked to fresh produce and reported to the United States Centres for Disease Control and Prevention (CDC) has increased in the last years (Bean and Griffin, 1990; CDC, 1990; CDC, 2000). Some of this increase is due to improved surveillance, but other factors may also come into play. A number of reasons have been proposed for this increased association of food-borne illness with fresh produce. Since the early 1970's, a significant increase in the consumption of fresh produce has been observed in the United States, presumably due, in part, to active promotion of fruits and vegetables as an important part of a healthy diet. If contamination levels were consistent, increased consumption of these foods should be expected to lead to greater numbers of illnesses over this time (CDC, 2000).

During this same period, there has been a trend toward greater consumption of foods not prepared in the home and an increase in the popularity of salad bars (buffets). Greater volumes of intact and chopped, sliced or prepared fruits and vegetables are being shipped from central locations and distributed over much larger geographical areas to many more people. This, coupled with increased global trade, potentially increases human exposure to a wide variety of food-borne pathogens and also increases the chances that an outbreak will be detected. Reasons for increases in food-borne illness in the summertime are not fully understood, although abusive temperatures and a higher consumption of fresh produce during the summer months are likely to play a role (CDC, 2000).

Microbial pathogens linked with outbreak of produce-associated diseases include Clostridium botulinum, Escherichia coli O157:H7, Salmonella species, Shigella species, Listeria monocytogenes, Staphylococcus aureus, Hepatitis A and Norwalk-like viruses. Outbreaks of salmonellosis have been associated with the consumption of sliced cantaloupe and watermelon and such contamination occurred during washing, during processing by an infected handler or cross-contamination through knives, cutting boards or hands. Escherichia coli O157:H7 and Salmonella species can survive and grow readily improperly stored melons. Outbreaks of E. coli have occurred as a result of consuming soft cheeses, hamburgers, salads, and fruits. Any food exposed to raw faecal matter is at risk of being contaminated.

Shigellosis is usually transmitted from person-to-person but may also occur by the consumption of contaminated water and foods such as fruits and vegetables that have received little or no heat treatment. Several outbreaks of shigellosis have been attributed to the consumption of contaminated raw vegetables (Beuchat, 1996). The outbreaks of Staphylococcus aureus in fruits and vegetables are mostly carried out by infected food handlers.