Depression in CFS is different than conventional depression.
- “Abnormal cerebral perfusion patterns in CFS subjects who are not depressed are similar but not identical to those in patients withdepressive illness…Depressed patients differed from those with CFS in having relatively less perfusion of the left prefrontal cortex.” 
- “Asymmetry (R > L) of tracer uptake at parietotemporal level is demonstrated in CFS as compared with Major Depression.” 
In 1999, while reading up on antibiotic protocols for Chronic Fatigue Syndrome, I came across the reports of Dr. Philip Bottero, MD, who was doing a protocol of multiple rotating antibiotics very similar to Dr. Cecile Jadin, MD. The unusual thing was he was treating patients with psychopathies and obtaining a high rate of remission (Clinique de la Résidence du Parc, 1986) and “Chronic psychopathologies associated with persistent rickettsia and/or similar germs (chlamydiae)” Acta Mediterranea di Patologia Infettiva e Tropicale, 1987) – “For 60 cases of diseases that are called “Psychic” associated with persistent rickettsiae; we have: 55 good and excellent result, 5 failure,”
This caused me to suspect many psychological disturbances were the results of bacteria — more recently, gut bacteria. But what does PubMed says?
- “several medications (pioglitazone, metformin, exenatide, atorvastatin, gram-negative antibiotics), which have traditionally been used to treat metabolic disorders showed a certain potential to treat depression in first randomized controlled clinical trials.” 
- “L. rhamnosus (JB-1) …these findings highlight the important role of bacteria in the bidirectional communication of the gut-brain axis and suggest that certain organisms may prove to be useful therapeutic adjuncts in stress-related disorders such as anxiety and depression” 
- “The concept that intestinal microbial composition not only affects the health of the gut, but also influences centrally-mediated systems involved in mood, is supported by a growing body of literature…. Bifidobacterium infantis…Probiotic treatment resulted in normalization of the immune response, , and restoration of basal NA concentrations in the brainstem.
- “B. infantis ….provides encouraging evidence in support of the proposition that this probiotic may possess antidepressant properties” 
- “39 CFS patients were randomized to receive either 24 billion colony forming units of Lactobacillus casei strain Shirota (LcS) [Yakult] or a placebo daily for two months…. also a significant decrease in anxiety symptoms among those taking the probiotic 
- “a multispecies probiotic containing Bifidobacterium bifidum W23, Bifidobacterium lactis W52, Lactobacillus acidophilus W37, Lactobacillus brevis W63, Lactobacillus casei W56, Lactobacillus salivarius W24, and Lactococcus lactis (W19 and W58) …. provide the first evidence that the intake of probiotics may help reduce negative thoughts associated with sad mood. Note: This is a Ecologic probiotic and should be available in Europe (soon)!
- “we compared the effects of two different Bifidobacteria on anxiety and depression-like behavior; an antidepressant was also used as a comparator… whereas B. longum 1714 induced antidepressant-like behavior in the tail suspension test.”  [PDF-2015]
- “these data suggest that B. longum 1714 had a positive impact on cognition and also that the effects of individual Bifidobacteria strains do not generalise across the species. ”  i.e. not any B.Longum will have this effect.