As I continue doing an in depth review of AI selected PubMed articles, I came across a study showing that hyperbaric chambers may actually make antibiotics less efficient and encourage antibiotic resistance. This is of concern because using hyperbaric chambers is popular in some communities based on logic such as: “increased oxygen level will kill off bad bacteria”
The sensitivity of selected bacteria to a range of antibiotics was tested under hyperbaric conditions used in saturation diving. The effect of hyperbaric helium and oxygen (heliox) on antibiotic stability and on induction of beta-lactamase was also determined. Increased resistance to penicillin (up to 23%) was shown by Staphylococcus aureus and to gentamicin (up to 46%) and rifampicin (up to 18%) by Escherichia coli and Salmonella typhimurium at 36 and 71 bar pressure. Exposure to 71 bar heliox did not affect antibiotic activity but increased the production of beta-lactamase in inducible S. aureus and Bacillus subtilis and production of beta-galactosidase in inducible E. coli. Increased resistance to antibiotics in saturation diving conditions can be attributed in some cases to the influence of hyperbaric pressure on induction mechanisms in bacteria. The experimental system devised for this work is suitable for more detailed examination of the influence of hyperbaric stress on antibiotic resistance and of its effect on induction mechanisms in general.The effect of antibiotics on bacteria under hyperbaric conditions. 
“The gram-negative bacilli generally gave reduced zone diameters in oxyhelium,”  – reduced zone means less effective.
In reviewing some studies, we find that it produces symptom relief especially where hypoxia is a factor (CFS/ME, FM, IBD, etc). The ‘Dave Berg’ school of thought ascribes this hypoxia to hypercoagulation in the tissue, often localized.
- “Hyperbaric oxygen therapy (HBOT) provides 100% oxygen under pressure, which increases tissue oxygen levels, relieves hypoxia and alters inflammatory pathways. ” 
- ” Recent evidence that hyperbaric oxygen inhibits the actions of certain cytokines, acts as an immune modulator and may help cognitive dysfunction has resulted in a re-examination of its potential role in rheumatic diseases. A case report of a lupus/scleroderma crossover patient is presented whose cognitive dysfunction improved after hyperbaric oxygen therapy.” 
- “For example, under normal conditions age-related macular degeneration involves oxidative stress and death of the retinal pigment epithelial cells. Hyperbaric oxygen therapy may exacerbate these processes…. Delivery of hyperbaric oxygen therapy may need to be modified or it may even be contraindicated in these cases. ” 
- Hyperbaric oxygen treatment for Parkinson’s disease with severe depression and anxiety: A case report. 
- Hyperbaric Oxygen Therapy for Alzheimer’s Disease .
- Adjunctive Hyperbaric Oxygen Therapy promotes successful healing in patients with refractory Crohn’s disease.
- Hyperbaric oxygen therapy for digital ulcers due to Raynaud’s disease. 
Here we encounter a treatment that appears to have good short term effects (i.e. reducing hypoxia) with the possibility of encouraging the underlying condition by promoting resistance or altering the microbiome in an unfavorable direction. A 1985 study raises questions about the impact of pressure on the cut microbiome which do appear to have been partially answered by a study this year:
- Gut fermentation seems to promote decompression sickness in humans .
- ” Thus, the result suggest that hyperbaric pressure is an important exogenous factor that strongly modulated the intestinal morphology and microbial ecology, and induced several gastrointestinal ailments during hyperbarism. ” 
We aimed to determine whether the composition of the fecal microbiota changes under hyperbaric conditions. In this study, we collected fecal samples from 6 healthy divers at three points during deep diving training (before, 2.1 MPa, end). The frequency of Clostridium cluster XVIII tended to be increased after compression. The frequencies of Clostridium cluster IV and subcluster XIVa were inversely correlated with that of Bacteroides. The compositional changes in the fecal microbiota exhibited interindividual variability. These findings suggest that hyperbaricconditions affect the fecal microbiota.Effects of hyperbaric conditions on fecal microbiota. 
Is it good or is it bad? It may depend on an individual circumstance.