The letter expresses the Food and Drug Administration's (FDA's) mounting concern following 19 outbreaks of E. coli 0157:H7 in the past decade, with one outbreak also reported in Minnesota last month.
The document, which provides guidance but does not set out new regulations, follows the failure of efforts by the California Department of Health Service (CDHS) over the past three years to successfully engage the lettuce industry and set out a comprehensive plan to address the issue of E. coli.
Infection with Escherichia coli serotype O157:H7 (E. coli), which was first described in 1982, has emerged rapidly as a major cause of bloody diarrhoea and acute renal failure. The infection is sometimes fatal, particularly in children.
Pre-washed salads encapsulate in many ways what modern consumers want: convenience, nutrition and safety. Unsurprisingly, sales topped $2.3 billion last year, according to market analyst ACNielsen.
The product has even been credited with boosting consumption of fruits and vegetables by making them easier and more attractive.
But the discovery of a potential food poisoning outbreak could change all that. Bacteria that cause food-borne illness can be destroyed by cooking, but most salads are served raw.
As a result, "the manner in which they are grown, harvested, packed, processed and distributed is crucial to ensuring that microbial contamination is minimized," said the FDA in the letter.
Contamination can occur at any point along the food supply chain. The E. coli strain is typically derived from animal fecal matter, which can come into contact with fresh produce at the farm level, primarily through floodwaters or high winds, or at the processing level, through poor hygiene conditions.
The FDA "strongly encourages" firms in the lettuce industry "to review their current operations" in order to ensure they are taking the appropriate measures to prevent contamination.
Last week the FDA issued recommendations for modernizing the current Good Manufacturing Practice (GMP)- a set of practices to help eliminate food-born illness risk factors. GMPs are currently mandatory for processors of fresh produce, but voluntary for packers.
According to the FDA, sanitation procedures are a key area for which regulations need to be tightened.
And last month, a final ruling requiring all manufacturers to register with the FDA was approved. The new law will allow the FDA to quickly locate food processors in the event of deliberate or accidental contamination of the food supply.
As result of the recent tightening up of procedures, the incidence of E. coli O157:H7 infections decreased 42 percent from 1996 to 2004. Campylobacter infections fell 31 percent, cryptosporidium dropped 40 percent, and yersinia decreased 45 percent.
Recent advances in food safety research are also enabling plant pathologists to gain insight into how dangerous human pathogens, such as strains of E.coli and Salmonella, can survive on fresh fruits and vegetables and what can be done to control future outbreaks.