The New York Times reports this morning that listeria can pose special challenges for artisan cheesemakers. Many make their cheese right on the farm, where the bad bacteria are right outside the cheese room door (or inside – dp). And small producers often lack the safety training and resources available at bigger companies.

For example, Sharon McCool got a shock last year as she prepared to begin selling the first batch of cheese she had made from milk produced at her family’s organic dairy, Rosecrest Farm, in Chehalis, Wash. State inspectors told her the Swiss cheese she had labored over for months was contaminated with listeria.

Tests showed that the bacteria was contained in a mixture of wine, water and salt, called a smear, that Ms. McCool had been brushing on her cheese each day as it aged. Without knowing it, she was dosing the cheese with listeria.

Ms. McCool threw out 900 pounds of cheese, changed some of her practices, got rid of the smear and started over.

Ms. McCool and her husband, Gary, have for years operated a dairy, where sanitation is paramount, so she thought she was well versed in food safety. But she now sees that she had overlooked some obvious pitfalls.

Before, Ms. McCool said, she had been so proud of her new business that she had often showed visitors around, possibly introducing listeria through their shoes or clothing.

Now, visitors are kept out. “Pretty much everything is really a whole lot stricter,” Ms. McCool said.

The number of small-scale, artisan cheesemakers has boomed in recent years but little or no formal training is required. Safety rules and enforcement also vary from state to state.

Soft cheeses — including brie, mozzarella and queso fresco — are more likely to become contaminated with listeria because they have a relatively high moisture content and low acidity, creating a more hospitable environment for the bacteria. In addition, listeria grows well at low temperatures, so even it can thrive even in a refrigerator.

The F.D.A. said it is inspecting both raw-milk and pasteurized cheese facilities for listeria, and recalls this year have been for both types of cheeses.

The agency also said it was reviewing the aging rules in light of evidence that the 60-day period might not be effective.