Mr Seymour said a consumer backlash was evident in anecdotal reports from suppliers, growers and retailers who said lettuce and carrot sales were falling.
He said investigating the outbreak was complex - the yersinia pathogen could spread from animal waste into water supplies, then into a vegetable growing area.
"Another part that complicates this is, the food safety sector which was part of MPI was disbanded through cost saving. So they don't have the resources to do it."
Labour food safety spokesman Damien O'Connor said the outbreak reinforced concerns about resourcing at MPI and "added fuel" to Labour's repeated calls for an independent food safety agency.
But an MPI spokesman said "dozens" of staff were working to locate the source. Retailer Foodstuffs, owner of New World and Pak 'n Save, said the most likely source of the contamination was farm-based.
"In exploring our own supply chain, should the contamination have come from Pams lettuce, we can confirm we have been able to narrow our investigation down to a single paddock from the supplier, Living Foods," the retailer said. The paddock has been quarantined.
Living Foods said it identified three paddocks producing lettuce during the time of the outbreak.
"Although none of our products have been identified as the source, as a precautionary measure these blocks have been retired and will not be used until MPI has completed its investigation," said Rannoch Goodwin, Living Foods general manager.
"If there is a single contamination source it cannot be associated with Living Foods, as a significant amount of illness was associated with Progressive stores, Countdown and First Choice."
Mr Goodwin said Living Foods had no connection with that supply or distribution chain.
There have been 127 confirmed cases of yersinia pseudotuberculosis.
