Initially, 16 people were diagnosed with the strain of E. coli O157 with the number of those affected rising to 18 earlier this month.
Health Protection Scotland (HPS) confirmed on Friday that another case has been diagnosed.
The cases developed symptoms between July 2 and 15.
HPS has been working with Foods Standards Scotland (FSS), NHS boards and local authority environmental health teams to investigate and manage this outbreak.
All patients, the majority of whom live in Scotland, are recovering at home, HPS said.
Officials are advising that – as a precaution – Dunsyre Blue cheese purchased between mid-May and the end of July with the batch codes C22 or D14 should not be eaten.