Observations on the growth and movement of Acinetobacter on semi-solid media

This study examined growth patterns of 29 Acinetobacter strains on semi-solid motility test media. Researchers observed two novel phenomena: surface swarming in 19 strains and the production of channels called "ditches" in 14 strains, particularly A. anitratus isolates. The ditching phenomenon involved fissures extending from the inoculation point that contained bacterial growth and fluid. Optimization studies showed that ditching and swarming occurred best at agar concentrations around 0.3% with 1% peptone at 30-37°C. Microscopic examination revealed that swarming involved chains of cells extending and lobes forming at the colony edge, without the flexing movements previously reported as gliding motility. Electron microscopy confirmed the organisms lack flagella, remaining non-motile. When artificial agar faults were created, Acinetobacter strains exploited and expanded these channels, while control organisms did not. The authors propose that swarming may represent gliding movement and could have taxonomic significance, though the capricious nature of ditching and the storage period required limit their practical diagnostic value.

Key findings

• Nineteen of 29 Acinetobacter strains exhibited surface swarming on semi-solid media, while 14 produced characteristic "ditches"—fissures containing bacterial growth and fluid
• Ditching and swarming were optimal at 0.3% agar concentration, 1% peptone, and temperatures between 30-37°C
• Electron microscopy confirmed Acinetobacter strains lack flagella, yet exhibited spreading movement on semi-solid media that may represent gliding motility
• Acinetobacter strains preferentially exploited artificial agar faults by expanding them into wide channels, unlike motile or non-motile control organisms
• The phenomena were inconsistent and required storage of strains for days to months before expression, limiting diagnostic utility