From what I get, "bacteroids" are bacteria-like organisms that show certain differences against regular bacteria, such as not being able to replicate.

"Bacteroides" is a specific genus among them, should always be used with capital "B", and may be used in singular form (as well as plural, meaning all species within the genus).

Batteroidi
Bacteroids

'Bacteroides' and 'Bacteroids'. They are confusing and difficult to differentiate.

From my research 'Bacteroides' and 'Bacteroids' are different bacteria types. The first is found in humans. The second in plants.


http://www.antimicrobe.org/b85.asp
At present, the Bacteroides fragilis group consists of ten species: B. fragilis (the most frequent isolate), B. distasonis, B. thetaiotaomicron, B. vulgatus, B. ovatus, B. eggerrthii, B. merdae, B. stercoris, B. uniformis, and B. caccae.


But in the following Italian file the two terms seem to be used with the same meaning.

http://www.biosteril.it/files/batteri/batteroidi.pdf

B. fragilis è il patogeno più rilevante: sebbene rappresenti solo l'1-2% della normale flora intestinale è la specie di batteroidi isolata nell'81% delle infezioni anaerobiche cliniche. B. fragilis non è dichiaratamente invasivo, tuttavia è in grado di concorrere nelle infezioni intra-addominali e, nell'eventualità, di distruggere le mucose delle pareti intestinali.

B. fragilis is the most significant pathogen: although only 1-2% of normal intestinal flora is the isolated species of bacteroids in 81% of anaerobic clinical infections. B. fragilis is not overtly invasive, however, is able to compete in the intra-abdominal infections and, in the event, to destroy the mucous membranes of the intestinal walls.
...

Ceppi di batteroidi isolati in alcuni pazienti con diarrea non diagnosticata sono stati riconosciuti come enterotossigeni (in grado di produrre specifiche tossine per le cellule della mucosa intestinale) e nei pazienti con meno di tre anni sono stati associati a crampi intestinali, vomito e feci sanguinolente. B. fragilis è l'organismo anaerobio isolato più di frequente nelle infezioni cliniche e, se non adeguatamente trattato, ha una mortalità del 60%.

Isolated bacteroid strains in some patients with undiagnosed diarrhea have been recognized as enterotoxigenics (which can produce specific toxins for intestinal mucosa cells) and in patients under three years of age have been associated with intestinal cramps, vomiting and bloody stools. B. fragilis is the most common anaerobic organism isolated in clinical infections and, if not adequately treated, has a mortality of 60%.


The following links refer to bacteroides

https://en.wikipedia.org/wiki/Bacteroides_fragilis

The B. fragilis group is the most commonly isolated Bacteroidaceae in anaerobic infections, especially those that originate from the gastrointestinal flora. B. fragilis is the most prevalent organism in the B. fragilis group, accounting for 41% to 78% of the isolates of the group.

Il gruppo B. fragilis è il Bacteroidaceae più diffuso nelle infezioni anaerobiche, in particolare quelle che derivano dalla flora gastrointestinale. B. fragilis è l'organismo più diffuso nel gruppo B. fragilis, che rappresenta il 41% al 78% dei (ceppi) isolati del gruppo.


https://microbewiki.kenyon.edu/index.php/Bacteroides
Classification
Higher order taxa:
Bacteria; Bacteroidetes/Chlorobi group; Bacteroidetes; Bacteroides (class); Bacteroidales; Bacteroidaceae; Bacteroides
Species:
Bacteroides caccae; Bacteroides distasonis; Bacteroides eggerthii; Bacteroides fragilis; Bacteroides merdae; Bacteroides ovatus; Bacteroides stercoris; Bacteroides thetaiotaomicron; Bacteroides uniformis; Bacteriodes vulgatus


http://www.anaesthetist.com/icu/infect/bacteria/anaerobe/Fin...
https://www.hopkinsguides.com/hopkins/view/Johns_Hopkins_ABX...
http://emedicine.medscape.com/article/233339-overview
http://emedicine.medscape.com/article/233339-treatment


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References for 'bacteroid' (they are found in Legumes):

https://en.wikipedia.org/wiki/Bacteroid
Bacteroid may refer to:
Bacteroides, a genus of Gram-negative, rod-shaped bacteria.
A symbiotic form of the nitrogen-fixing bacteria, Rhizobia.

http://medical-dictionary.thefreedictionary.com/Bacteroids
bacteroid
Any of various bacteria, especially those of the genus Rhizobium, that carry out nitrogen fixation in the root nodules of leguminous plants and often change markedly in size and shape after they enter the roots.
bacteroid
1 pertaining to or resembling bacteria.
2 a structure that resembles a bacterium. Also bacterioid bacteroidal, bacterioidal, adj.
bacteroid
1. resembling a bacterium.
2. a structurally modified bacterium.

https://en.oxforddictionaries.com/definition/us/bacteroid
A bacteroid organism or structure, especially a modified cell formed by a symbiotic bacterium in a root nodule of a leguminous plant.

https://www.merriam-webster.com/dictionary/bacteroids
Definition of bacteroid
:an irregularly shaped form of a nitrogen-fixing bacterium (such as a rhizobium) found especially in root nodules of legumes

https://microbewiki.kenyon.edu/index.php/Legume-Rhizobium
https://academic.oup.com/femsre/article/37/3/364/584543/Mole...
http://www.pnas.org/content/114/19/5041.abstract
http://www.plantphysiol.org/content/154/3/1541
http://jb.asm.org/content/172/8/4295
http://www.microbiologyresearch.org/docserver/fulltext/micro...
https://www.ncbi.nlm.nih.gov/pubmed/10607628
http://www.tandfonline.com/doi/abs/10.1080/713610850


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I assume that there are some dictionaries that make mistakes or are less accurate, because the following reports 'bacteroid' as "any rodlike bacterium of the genus Bacteroides, occurring in the gut of humans and animals".

http://www.dictionary.com/browse/bacteroid
http://www.thefreedictionary.com/Bacteroids







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Note added at 19 hrs (2017-10-02 20:42:13 GMT)
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Note that I answered to the kudoz question "Italian term or phrase: batteroidi' = 'bacteroids'.
Then I answered to the second part of the question: 'I find "bacteroides" as well as "bacteroids" online when I research this term ... the translator used "bacteroides". Is there a difference? Which is the correct translation?'.
At this part of the question I answered: '
From my research 'Bacteroides' and 'Bacteroids' are different bacteria types. The first is found in humans. The second in plants.'
Then I detailed my research and is evident that because I wrote 'The following links refer to bacteroides. https://en.wikipedia.org/wiki/Bacteroides_fragilis The B. fragilis group is the most commonly isolated Bacteroidaceae in anaerobic infections, especially those that originate from the gastrointestinal flora'.
Then I wrote 'Definition of bacteroid: an irregularly shaped form of a nitrogen-fixing bacterium (such as a rhizobium) found especially in root nodules of legumes'. So, if bacteroids are found in plants, while bacteroides are found in humans, as I said before, it's implied that Bacteroides are the responsible of 'acute diarrhea', while bacteroids are not.