I'm playing on a low configuration and of course I need Optifine to play. As the latest version of Mekanism has been released (not sure if it was the latest, is on version 1.7.10-7.1.1.105 the problem occured) and now, some machines textures seems buggy and invisible.
Can not understand why and how, so I took screenshots:
Without Optifine:

And with Optifine:

Can you give me your optifine settings?

It's a problem with rendering.
Maybe optifine is blocking or disabling rendering?

I think it is a problem with Optifine, not wih Mek.
P.s.: If you have a low configurations, disable fancy cables rendering in Mek's config file.

--Edit--

Some fixes:
1.Go to this video and try this fix: http://www.youtube.com/watch?v=VTXaSdsE0aI
2.Try to change Chunk Loading in Video Settings.

@Exy-mat use Forge 1232 at most until Optifine updates

My Optifine settings:
ofRenderDistanceChunks:2
ofFogType:3
ofFogStart:0.2
ofMipmapType:0
ofLoadFar:false
ofPreloadedChunks:0
ofOcclusionFancy:false
ofSmoothFps:true
ofSmoothWorld:true
ofAoLevel:0.0
ofClouds:3
ofCloudsHeight:0.0
ofTrees:1
ofGrass:1
ofDroppedItems:1
ofRain:3
ofWater:1
ofAnimatedWater:0
ofAnimatedLava:0
ofAnimatedFire:true
ofAnimatedPortal:true
ofAnimatedRedstone:true
ofAnimatedExplosion:true
ofAnimatedFlame:true
ofAnimatedSmoke:true
ofVoidParticles:true
ofWaterParticles:true
ofPortalParticles:true
ofPotionParticles:true
ofDrippingWaterLava:true
ofAnimatedTerrain:true
ofAnimatedTextures:true
ofAnimatedItems:true
ofRainSplash:true
ofLagometer:false
ofAutoSaveTicks:40000
ofBetterGrass:3
ofConnectedTextures:1
ofWeather:false
ofSky:true
ofStars:false
ofSunMoon:true
ofChunkUpdates:1
ofChunkLoading:1
ofChunkUpdatesDynamic:false
ofTime:0
ofClearWater:false
ofDepthFog:false
ofAaLevel:0
ofProfiler:false
ofBetterSnow:false
ofSwampColors:false
ofRandomMobs:false
ofSmoothBiomes:false
ofCustomFonts:true
ofCustomColors:true
ofCustomSky:false
ofShowCapes:false
ofNaturalTextures:false
ofLazyChunkLoading:true
ofFullscreenMode:1920x1080
ofFastMath:true
ofFastRender:true
ofTranslucentBlocks:1

I'm using Forge 1236, so what's the difference with the 1232 ?

N.B: I'm not using the multi-core chunk loading because I have one single core :(

Try Forge 10.13.2.1230, download Standard Optifine, not ultra (Only for a test!) and download latest Mekanism (1.7.10-7.1.1.127).

You can try also older versions of Optifine standard. :)

so what's the difference with the 1232 ?

One rendering bug was fixed in 1233. Of tries to fix it again and fails.

Thanx SzYmCi, you solved the problem. It's the version of forge who is broken. With Optifine HD U A4 and Forge 1230, all the texture "magically" reappeared :

And, thanx too laz2727 and grimallq ;)

You're welcome!