Subspecies is the lowest rank dealt with by the Bacteriological Code. The rank ‘subsp.’ is included in the name, as for plants:

Salmonella enterica subsp. enterica

Other ranks include biovar (bv.), serovar (sv.), pathovar (pv.), phagovar and special form (f. sp.). These taxa may be designated in a number of ways, including by Latin words, vernacular names, numbers, letters or formulae:

Xanthomonas translucens f. sp. cerealis     Staphylococcus aureus phagovar 42D

The names of serovars begin with an initial capital letter and are shown in roman type. Include the word ‘serovar’ or ‘sv.’ with the species name the first time the name occurs in the text:

Salmonella enterica serovar Typhimurium     Salmonella enterica sv. Typhimurium

Afterwards, you can omit the species name and ‘serovar’ or ‘sv.’, but the genus name should remain in full:

Salmonella Typhimurium   not   S. Typhimurium