Jennings' place is under scrutiny following a hard series, although he seems to have earned a stay of execution with a 48 in the second innings of England's 239-run win at The Oval.
That left time for just four overs before lunch - and Anderson did not miss his cue with the nearly instant dismissal of Elgar, a centurion at The Oval only five days ago.
While he was laid low, and even spent a night in hospital on a drip mid-match, South Africa went down by 239 runs and can only draw the series at best at Old Trafford.
Root made 52 of the hosts' 260 for six day one at Emirates Old Trafford, inking his name into the annals as the third youngest batsman to pass 5,000 Test runs and equalling John Edrich's English record of 10 successive games with a half-century score.
South African captain Faf du Plessis would have been concerned when Keshav Maharaj was forced off for treatment early in his spell but England continued to be pegged back as they ended the morning session on 67-1 at a run rate of just 2.31.
Hashim Amla made a typically stylish 30.
Nightwatchman Toby Roland-Jones fell for four when he drove Rabada to Temba Bavuma at cover-point.
South Africa wicketkeeper Quinton de Kock had a shocker when he chose to let Root's edge sail past him in the evening session but earlier in the day he moved to a century of Test dismissals in classic fashion.
Bavuma narrowly made his ground on 26 after a mix-up over a single to mid-off and Moeen's failure to break the stumps at the first attempt from Alastair Cook's throw.
But Bairstow, playing some handsome strokes, put on a 50-run partnership with last man Anderson - with 46 of those runs coming from Bairstow.
Then, from what turned out to be the final ball of the day, Stokes held a stunning, plunging, left-handed catch to remove Kagiso Rabada off Broad.
England, who'd won the toss, resumed on 6-260 after Kagiso Rabada had yorked the unsafe Ben Stokes for 58 late on Friday.
Bairstow, on 4, was given not out, out and not out again by a combination of Kumar Dharmasena and the TV umpire, Joel Wilson, after an inside-edge off Maharaj was adjudged to have fallen millimetres short of Dean Elgar at slip.
The 25-year-old says he was prepared for the late call-up following the injuries to Chris Morris and Vernon Philander, and has been preparing and practising like he was going to play throughout the week.
In the next 33 deliveries England scored no runs and lost Westley too, Rabada forcing the mistake and De Kock flinging himself to the right to pouch a superb catch.
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