One of the popular ideas is that decline in methyl-directed mismatch repair 
(MMR) in carbon-starved bacteria might facilitate occurrence of stationary-phase 
mutations. We compared the frequency of accumulation of stationary-phase 
mutations in carbon-starved Pseudomonas putida wild-type and MMR-defective 
strains and found that knockout of MMR system increased significantly emergence 
of base substitutions in starving P. putida. At the same time, the appearance of 
1-bp deletion mutations was less affected by MMR in this bacterium. The spectrum 
of base substitution mutations which occurred in starving populations of P. 
putida wild-type strain was distinct from mutation spectrum identified in 
MMR-defective strains. The spectrum of base substitutions differed also in this 
case when mutants emerged in starved populations of MutS or MutL-defective 
strains were comparatively analyzed. Based on our results we suppose that other 
mechanisms than malfunctioning of MMR system in resting cells might be 
considered to explain the accumulation of stationary-phase mutations in P. 
putida. To further characterize populations of P. putida starved on selective 
plates, we stained bacteria with LIVE/DEAD kit in situ on agar plates. We found 
that although the overall number of colony forming units (CFU) did not decline 
in long-term-starved populations, these populations were very heterogeneous on 
the plates and contained many dead cells. Our results imply that slow growth of 
subpopulation of cells at the expenses of dead cells on selective plates might 
be important for the generation of stationary-phase mutations in P. putida. 
Additionally, the different survival patterns of P. putida on the same selective 
plates hint that competitive interactions taking place under conditions of 
prolonged starvation of microbial populations on semi-solid surfaces might be 
more complicated than previously assumed.