Difference between bacteria and bacteroids?
Bacteroids are the endosymbiotic forms of rhizobia (in the root nodules of leguminous plants).



Löhnis &amp; Hansen (1921) J Agric Res 20: 543-556; emphasis mine:


  It is not superfluous to emphasize once more that persistence in
  calling these forms "bacteroids" is by no means to be recommended.
  They are true bacteria, not foreign bodies looking like bacteria, as
  Frank's pupil Brunchorst erroneously believed. To speak of a
  "bacteroid" growth of bacteria is no less absurd than it would be to
  speak of a "fungoid" growth of fungi.


https://naldc.nal.usda.gov/download/IND43966363/PDF



Gerson &amp; Patel (1975) Appl Microbiol 30: 193-198; emphasis mine:


  Rhizobium is an aerobe, existing as a free-living bacterium in soil or
  as a partially anaerobic symbiont in the root nodules of leguminous
  plants. It is in the symbiotic form (bacteroid), which is frequently
  morphogenetically different, that nitrogen fixation takes place.


https://www.ncbi.nlm.nih.gov/pmc/articles/PMC365797/

MacKenzie et al. (1973) J Bacteriol. 113: 387-393; emphasis mine:


  [C]hanges [accompany] the transformation from the free-living rod
  forms to the nitrogen-fixing bacteroid forms. (...) During the
  transformation into the bacteroidal state the wall decreased in
  thickness, became less rigid, and developed a particulate surface.


https://www.ncbi.nlm.nih.gov/pmc/articles/PMC251641/