It is important to be able to distingish food poisoning from food spoilage. The former is when food is eaten which looks normal, smells normal and tastes normal. Hence you eat enough to make you ill from the ingested pathogens or toxins. Spoiled food does not normally cause food poisoning because it is rejected by the consumer before ingestion. In order to help determine if food is spoiled pleae note the list below (yes I have a sense of humour):

Food Spoilage Test (sense of humour required...) 
EGGS: When something starts pecking its way out of the shell, the egg is probably past its prime. 
MAYONNAISE: If it makes you violently ill after you eat it, the mayonnaise is spoiled. 
FROZEN FOODS: Frozen foods that have become an integral part of the defrosting problem in your freezer compartment will probably be spoiled -- (or wrecked anyway) by the time you pry them out with a kitchen knife. 
EXPIRATION DATES: This is NOT a marketing ploy to encourage you to throw away perfectly good food so that you’ll spend more on groceries. Perhaps you’d benefit by having a calendar in your kitchen. 
MEAT: If opening the refrigerator door causes stray animals from a three-block radius to congregate outside your house, the meat is spoiled. 
BREAD: Sesame seeds and Poppy seeds are the only officially acceptable "spots" that should be seen on the surface of any loaf of bread. Fuzzy and hairy looking white or green growth areas are a good indication your bread has turned into a pharmaceutical laboratory experiment. 
FLOUR: Flour is spoiled when it wiggles. 
SALT: It never spoils. 
LETTUCE: Iceberg lettuce is spoiled when you can’t get it off the bottom of the vegetable crisper without Comet. Romaine lettuce is spoiled when it turns liquid. 
CANNED GOODS: Any canned goods that have become the size or shape of a softball should be disposed of. Carefully. Very, very carefully! 
RAISINS: Raisins should not be harder than your teeth. 
POTATOES: Fresh potatoes do not have roots, branches, or dense, leafy undergrowth. 
CHIP DIP: If you can take it out of its container and bounce it on the floor, it has gone bad. 
EMPTY CONTAINERS: Putting empty containers back into the refrigerator is an old trick, but it only works if you live with someone or have a maid. 
UNMARKED ITEMS: You know it’s well beyond prime when you’re tempted to discard the Tupperware along with the food. Generally speaking, Tupperware containers should not burp when you open them. 
GENERAL RULE OF THUMB: Most food cannot be kept longer than the average life span of a goldfish. Keep a goldfish nearby your refrigerator to gauge this. 

More seriously....
Ready-to-eat food guidelines and surveillance studies
Since the publication of my 2000 book the PHLS (UK) guidelines for ready-to-eat foods (Table 8.20, p333) has been modified already (Such is the pace of food microbiology and the need for web page up-dates!). The ’Aerobic plate count’ is now referred to as ’Aerobic colony count’’ and the Clostridium perfringens Satisfactory level is 20/g. The units for the Table should be per gram unless otherwise stated (ie per 25 g as for Salmonella etc.

The reference for the new PHLS ready-to-eat guidelines is Anon (2000) Guidelines for the microbiological quality of some ready-to-eat foods sampled at the point of sale. Communicable Disease and Public Health 3, 163-167. You can also download a .pdf file version by going to the PHLS (UK) web site and clicking on ’Food Sampling’.

The USA are collecting baseline data of microorganisms in food (FSIS). Meanwhile there are a number of microbiological surveillance studies that have been published on various ready-to-eat foods, assessed according to criteria such as the PHLS (UK) RTE guidelines. These include:

RTE surveillance studies 
Ice cream 
Stuffing (Richardson & Stevens 2003 J Appl Microbiol 94, 733-737) 
Salads 
Organic vegetables (Sagoo et al. 2001 Lett Appl Microbiol 33, 434-439) 
Meat and meat products (Joint Food Safety and Standards Group, UK, Number 9) 
Burgers (Little et al. 2001 Commun Dis Public Health 4, 293-9) 
Cooked rice (Nichols et al. 1999 J Food Protect 62, 877-882) 
Chicken sandwiches (Little et al. 2002 Commun Dis Public Health 5, 289-98) 
Foods from sandwich bars and take-aways 
Quiche (Gillespie et al. 2001 Commun Dis Public Health 4, 53-9) 
Salmonella and Campylobacter contamination of fresh & frozen chicken 
Aflatoxins 
These studies help to shows the proportion of foods containing pathogenic bacteria (not viruses or toxins), and contribute towards the microbiological risk assessment of such foods.

Dairy products, lactic acid bacteria and probiotics
Lactic acid bacteria have been used for centuries to preserve various food products (Overview). 

More recently there has been considerable interest in the use of lactic acid bacteria and related organisms in the production of probiotics (p134). Below is a list of web sites some of which are commercial which sell probiotic cultures for a rang of purposes. Look trough the sites and decide which tests you would carry out to determine the efficacy of these products. Firstly consider the number of organisms that would survive the stomach (lactic acid bacteria will be acid resistant) and the number of bacteria already colonising the intestinal tract.

In the book I have introduced the potential applications of DNA arrays, etc as I believe modern food microbiologists should be aware of these developments, even if they do not have access/direct use of them. The complete genomes of many important foodborne pathogens have now been released and are listed here. Additionally the genomic sequence of lactic acid bacteria is under investigation and recently Lactococcus lactis has been published (genomic sequence) and this may assist in our understanding of probiotics. 

Probiotics 
WHO draft guidelines on the evaluation of probiotics (May 2002) 
Lactic acid bacteria description 
Lactobacillus description 
Yakult 
Actimel 
Wakunaga products 
Probiata 
Natren 
Yogurt-probiotics 
Candida and probiotics 
Living Well with Probiotics 
Custom probiotics 
Metagenetics-probiotics 
Dr Ohhira’s original probiotic culture ! 

Food additives are used for a variety of purposes, including preservation. A useful site which can be used to search for data on food additives has been compiled by the Nordic Working Group on Food Toxicology (Nordic Food Additive database). 

If you are looking for useful pictures of food matrix and micro-organisms a useful site is Foods under the microscope

A couple of web sites I have just picked up on eggs are good at explaining the egg structure and associated microbial hazards:

Egg sites 
Egg guide 
CDC site on Salmonella Enteritidis and eggs